Friday, 30 October 2009

Quality over Quantity

It can be very hard finding the quality of fish that I like to sell especially on the back of a windy week. Bad weather over a weekend moves the wholesalers’ and the exporters’ bottomless pocket buying power into Tuesday and sometimes Wednesday leaving the smaller boys scrapping around for the remains. Wouldn’t it be lovely to have a demand for 52 day aged sea bass, but unfortunately this is what makes fishmongery so much more difficult than butchery – it needs to be in and out in one day. Ok by the look of some other establishments fish this is not always the case but it is certainly the principal I try to adopt.

It is, however, lovely to see, and especially on a Friday, some high quality fish from some of the most reliable boats in the fleet. The fishing port of Plymouth (the fine town where I read my fisheries science degree) now has an updated re-built busy market with a near faultless on line Dutch auction. Many boats have now chosen to land here because of the facilities. The lemon sole pictured were landed by the ever reliable Plymouth registered ‘Our Endeavour’.

Of course my favourite fish are landed into the quaint fishing port of Newlyn and although their soon to be updated market is a little dated it does have the glamour of a shout auction which gives a lovely historical feel. More importantly though is the Newlyn registered Scorpio, the most reliable Red Mullet boat in the fleet. Pristine fish (pictured) on every landing incurs much interest and huge demand.

A fish that always creates an interest are hook and line caught mackerel. “The only way to eat them is straight out of the sea” I hear. All well and good but there is not much of a coastline in London so you will have to rely on some of the St Ives registered hand-liner the ‘Janet Ann’ to supply the best in London – and they are the best!!

The skippers of these boats lead the way when it comes to looking after their catch. They clearly realise the higher the quality when reaching the market, the higher the market price. I see Icelandic lemon sole and small trawled red mullet and tiny watery Scottish mackerel (which I would be embarrassed to sell) at Billingsgate that are less than half the price, however, I never whinge when paying through the nose for the quality of fish from my favourite boats. Twice the price maybe, five times the quality certainly.

Monday, 19 October 2009

Reform of the Common Fisheries Policy - Maybe?

Ever since the cinema premier of the hugely revealing documentary 'The End of The Line', which exposes frailties in certain fisheries around the world, everybody has become an expert. The politician, the chef, the angler, the greenie, the organic and so on all now think they have a full insight into the plight of the worlds fisheries so much so they are polished in all aspects of fisheries management. Apparently the fisherman, the fish wholesaler and the fishmonger are clueless and have no understanding about the current situation of certain depleted species and are only interested in capitalising on the profits that can be made from fish. I can't speak for others but in my case this could be no further from the truth. With the TV premier of this documentary imminent i ask those who will watch it for the first time to listen to the words and not just be memorised by the shock factor pictures unveiling in front of your eyes. After watching we will all have an opinion, whether structured or not, however, few people have offered me any better suggestions on how to solve this problem other than 'talking about' buying pollack instead of cod or not buying blufin tuna that isn't available for them to buy anyway. Why? Because the documentary offers no clear insight or understanding into the subject of fisheries. Please enjoy some of my opinions, based around our more local situation of the European Union, and maybe it will also give an insight into how complex the fisheries world really is.

It could be argued that Britain and Europe must have a sustainable fishery to meet the requirements of consumer demand and coastal employment. In the majority of cases fish are common property, but can they still be treated as a free resource? Or will this continuation cause the extinction of many species? Fish resources are not only limited, but are increasingly insufficient, pointing to an argument that fish resources can no longer be treated as free. To implement this is a challenge; if tackled incorrectly the collapse of Eco-systems, entire food chains and huge economic hardships may result. Organisations throughout the world have been established to undertake these problems; Europe is no exception.

The common fisheries policy originated from the accession of three countries to the European union. With Great Britain, Denmark and Ireland’s application, the existing member states felt it was necessary to compile a policy concerning the fisheries in EU waters. This policy would attempt to protect and manage the existing and new member’s rights. The new members joined in 1973 with the basic foundation of the policy being laid down on November 3rd 1976; this coincided with the introduction of a 200-mile fisheries limit by the EEC. This agreement included management techniques in the form of total allowable catches (TACs) and individual quotas for each member state for each of the main fish stocks.

One problem that arises is the apportionment of TACs and quotas. The method of allocation is still based on an agreement reached in 1980 relating to relative stability of states; member states maintain their share of a TAC over time. Interestingly, it must be noted that fish stocks fluctuate, effort levels vary, technology improves and demands change, but TAC allocation methods stay the same. This approach seems somewhat static and although allocation is modified yearly within TACs it is probably the case that when they are at their lowest there are to many boats chasing to many fish (this may also be the case when set at highest levels as it is dependent on exploitation levels).

People in charge of setting TACs have on occasions been inexact, leading to collapses of certain stocks. The problem stems from data availability and the inaccurate scientific techniques used in quantification. Although indirectly associated with the common fisheries policy, assessment methods need to improve. More accurate data is essential and bias needs to be eliminated from mathematical models, it may be fair to assume that in submission of figures these problems may not be taken into consideration for political reasons. The continued setting of incorrect TACs may eventuate in a failure to conserve stocks (the key rationale behind the common fisheries policy), hence undermining the whole process in the eyes of fishermen. These and other techniques have essentially failed to safeguard fish stocks. This leads to the long-term future of fishing industries being unpredictable as stock depletion has failed to ensure a sustainable fishery.

The resident allocation of quotas could have become redundant in 2002, when the question of open access was addressed. It would seem that reform was fundamental in this area otherwise the community would continue with 10-year recesses resulting in no denouement of the issue. The engagement of free access was of course unrealisable, as this would require unanimity amongst member states which is highly unlikely. A more viable notion is regional or national control. Allocation would stay adequately constant, but regionalising of management and control will assist against the enigmas of enforcement.

Enforcement of laws and regulations are arduous and costly and with some fishermen just covering their variable costs, there will always be falsification in the system; it could probably be said that abuse is rife. Abuse in the system will proceed whilst fishermen have to contend for dwindling fish resources. So would an abatement of capacity in the fishing fleet resolve these problems? The accession of Spain and Portugal in 1986 put an augmenting burden onto Europe’s total catching fleet. Technically Spain is part of Europe, but at this stage they were not union members, so prior to their accession the majority of their vessels were fishing in distant non-EU waters. Once they had joined (after prolonged discussions), Europe’s fleet had the added problem of compensating for some of Spain’s vast fleet (17300 vessels). This expanded competition between vessels meant far too much power pursuing too few fish.

With continued accession of European countries coinciding with deficient polices, reform seems the only option. The largest consideration entails what areas can be amended and how many disturbances it may cause to the fisheries sector. It could be argued that increasing the level of officious bureaucracy into a system that is already over-bureaucratic, is unnecessary and definitely unwanted by the fishermen. This leads to the opinion that the common fisheries policy is politically driven. Members will negotiate between themselves to gain the best deal for their country, their fisherman, or more cynically their political party. The problem is the lower priority, given to conservation and long-term sustainability throughout negotiations.

If excluding the political interactions that obviously play such a large role in policy making, the reform must then directly reflect on the fleet, the main contributors to stock reduction. If through reform, the cheats in the system can be reduced or removed, the policies, in theory should work with greater efficiency. Convincing fisherman that regulations set by ‘armchair’ politicians are indeed beneficial, may be the most strenuous task. Being told how much you are authorised to catch after years of exploiting a free resource will obviously cause dissatisfaction in fishing sectors.

Allowing fishermen their own rights to the fish rather than be granted them by a European government may increase faith in the policies and may give good reasons to subscribe to their success. Allowing fisherman to buy into and sell out of these schemes would help to give them a sense of ownership. This could stop the immense problems of discarding, illegal net sizes and immoral bycatches as fishermen would be cheating a system they consider their own. Essentially, the system would become self-enforcing, thus taking the pressure from the already inadequate enforcement agencies. On the subject of discards it must be conceived that this behaviour is not only influenced by market opportunities, but in some cases it is in the direction of conservation. Obviously, it cannot be classed as conservation, therefore policies concerning this problem are unquestionably in need of reform.

These suggestions coinciding with developments of satellite tracking will aid the uncertainty of enforcing policies. By tracking catches to the point of sale, assurances that fish obtainable on the market were legally caught as part of a sustainability management scheme can be given.

The more extreme method of reform would entail a reduction in fleet capacity across the board. This is a feasible option if befitting compensation is given to members who choose to vacate the fleet and deploy their maritime skills elsewhere. The problem with this policy is many traditional fishing communities have few if any alternative sources of employment thus a return to politically sensitive issues. Decommissioning is an unavoidable evil. There will be no long-term future for the fishing industry unless fishing stocks are offered a chance to recuperate and, when recovered, the fish are harvested at a sustainable level.

This decommissioning may be prerequisite, but the techniques used are somewhat prejudiced. It must be unacceptable that states are decommissioning vessels, facilitating supplementary fishing opportunities in their waters for the fishing industry of their European partners. Also vast amounts of money are paid into cohesion funds to help the upgrading and upkeep of other countries fishing fleets. It may also be the case that some countries are paying more for the upkeep of partner’s vessels than to their own fisherman to eradicate theirs. When considering these points it is of no surprise that some members of the fishing industry object to the common fisheries policy.

Essentially, a reduction in fleet size will not initially enlarge stock sizes. If the TAC system remains, the quota of fish targeted by the decommissioned boat will be shared amongst other vessels. If this process continues it would be fair to say that socio-economic impacts will result, but the stock of fish is unlikely to be conserved. This argument leads directly into quota reductions, a highly sensitive issue that without accompaniment of decommissioning, would bankrupt fishermen without any compensation. Once a vessel has been scraped the equivalent quota should follow the same route.

To conclude, areas of the common fisheries policy require reform if sustainable fisheries are to be met. Fishermen require more stability and understanding of the regulations that are being enforced. This may be done through implementing schemes enabling them to buy into the rights of fish stocks and in turn reduce the obvious abuse to the system. Essentially, a policy that enforces itself will be more effective and may reduce problems of unnecessary bycatches and gear usage. A reduction in fishing effort will reduce the competition and catch per unit effort, making the fishery more efficient and less likely to hoard poachers. This will only work if quota levels are re-assessed alongside decommissioning. The communities are required to offer adequate compensation for those choosing to leave the industry for retraining in other sectors. The calculation and setting of total allowable catches need to be 1) more dynamic instead of the exceedingly static method deployed at the moment and 2) more accurate.

Finally, the issue of open access requires settlement with control being passed over to regional and national authorities, instead of being centralised in Brussels. This should ameliorate enforcement and management strategies that are currently inadequate.

Sunday, 18 October 2009

Italy's Island of Fantastic Fishes

I admit that i have an obsession with fish so with every visit to anywhere i will seek out the aquarium, the fish market, fishmonger or fishing port. On my recent honeymoon to the Island of Ischia i was a little spoiled as this area of Italy really depends on fish to fill their busy restaurants.

Externally this could be the prettiest front to a fishmongers i have ever seen. It was situated in Ischia Ponte just before the headland to the castle. It really was an eye catcher, but sadly the fish inside, mainly farmed Orata and Spigola were of fairly poor quality.

At the start of the causeway that leads to the castle i found a concrete pontoon that yielded a small collection of fishing boats selling their fare in some of the most breathtaking surroundings.

This fisherman obviously takes care of his catch as his fish were carefully kept in salt water that he continually re-freshed. In this picture he was selling a large red octopus to a delighted Signora. It was still alive and about 1.2kgs (my estimate) in size. Price was about 20 euros per Kilo. The middle blue tub was full of cuttlefish.

Our man also had a nice collection of rock dwellers such as wrasse, weavers and small scorpion fish.

The sale comes to an end but what was noticeable was the lack of bargaining. The fishermen valued their product and no-body questioned their prices.

Mending their small tatty nets. Can't believe they ever catch anything with their gear.

I stumbled across a busy food market in a town called Casamicciola that had a small covered fish stall. Bear in mind the temperature was approaching 27 and not a cube of ice was to be seen!!

The stall, albeit not the best quality, had some very interesting species. My first spot of the famous gambero rosso, huge cuttlefish and a box of garfish.

This was the largest fish stall i found, again void of ice. It was also situated in the town of Casamicciola.

He was selling a large, may be too large, selection of fish with some impressive swordfish, rascasse and tuna and some not so impressive bass, bream, mullet and so on. We had beautiful weather whilst on Ischia with not a cloud in the sky. After visiting this mongers i realised he had all the clouds pasted to his fishes eyes! His shop needed a clean up too. That aside displays like this allow you to see what local fish species are available.

These two pictures are common site in Ischia Porto where the busy restaurants are competing for tourist diners. Among the amazing swordfish, scorpion fish and red bream are defrosting langoustine, farmed turbot and old farmed bream. Why???

Saturday, 17 October 2009

Effects of Electricity on Fish

A number of years ago i participated in a fish survey that was undertaken on a southern section of the grand union canal. It gave me the perfect opportunity to gain some photographic evidence of fish reaction to electrical fields. I wrote this short piece explaining how fish then react when in contact with these electrical fields. I did not delve to deeply into the rather confusing areas of theoretical reactions of fish as this rather is long-winded.

There are three types of electrical current available to fisheries scientists - uninterrupted direct current; pulsed direct current; alternating current.

Three keys stages are witnessed when fish are exposed to electrical fields. The initial reaction is commonly known as 'excitation' and all current types result in similar responses. Fish show restlessness and begin to shudder as they come into line with, and then lie along, the lines of the force.

The second stage is known as 'electrotaxis' and does tend to vary slightly between current types. Uninterrupted and pulsed DC are, however, similar. The fish begin to move towards the anode and away from the cathode. When the anode is reached the fish becomes immobile with their head facing the anode.

The slight difference with pulsed DC is that if the pulse is set to frequent some larger fish can succumb to muscular cramps. The reaction of fish when using DC in the headed section of 'electrotaxis' is sub-headed to galvanotaxis . Conversely, in the same section, but applying AC, is known as oscillotaxis. This area of fish reaction varies from that of DC. The fish become oppressed into occupying a position along the euipotential (central between positive and negative).

The final section of fish reaction is 'electronarcosis'. With DC this sub-section is known as galvanonarcosis. Fish become dazed and immobile and show little or no response to alternative stimuli. At this point the fish will cease to breathe for short periods. Depending on species, some will sink (eels, bullheads, initially carp and zander) and others will float (bream, pike, carp after a short period and zander after a couple of minutes but float for a long period there after). This is a critical point as long periods in the field can cause death. Conversely, shorter periods are ideal as fish will recover and return to normal.

The effects of AC are known as oscillonarcosis and are very similar to galvonarcosis, however, the physiological effects are much greater thus the reason why DC is now more commonly used.

The difficulty in gaining accurate photographs on a working boat means that the following images only fall between the categories of the final stages of galvanotaxis through to complete galvanonarcosis.

These three images are good examples of multiple fish reaction to direct current . The top image shows a large crucian carp clearly under the state of electrotaxis. By looking at the water pattern this shows the fish being pulled towards the anode (positive) headfirst. The middle picture shows some bream in a state of electronarcosis - visual state of immobility and beginning to float to the surface. The bottom image shows the result when counteracting a shoal of fish. All responses are shown in this picture. All pictures show the right arm of the electrofishing boom boat and a fisheries technician working an extra hand held anode to increase the fishable width and efficiency.

Thursday, 15 October 2009

Just another Thursday

I deem myself very lucky to have started my fishmonger career at the very top. Being situated in SW3 I am able to deal with the highest quality of produce on a day basis. There are many lines covered by regular fishmongers that I have never sold or simply avoid selling. This is not because they are not nice, in fact some are delicious, but at the end of the day we are a business and need to sell everything to turn a profit; waste is criminal. This blog post is really a photo call of my Thursday delivery highlighting the quality that can be achieved across the board. People tell me that Tuesday and Friday are the only days to buy fresh fish. I disagree. I thought it would be nice for all to see what quality is available if searched for in London.

As with any of the best fishmongers in London I source my highest quality fish from the West Country. Plymouth, with its computerised Dutch auction system, Brixham with its draw of hungry wholesalers and Newlyn with its rich exporters all encourage the best selection of fisherman to land they fare as the prices will rarely be topped.

Although bought through a London wholesaler my scallops are again from the West Country and come via Brixham. They look as good as they taste.

Tagged Cornish Bass,, landed in Newlyn really are the cream of the crop. Other excellent sources, such as the English Channel, offer high quality, but the MSC accredited Bass hand line fishery is rarely surpassed

The second important MSC accredited line fishery is for the Atlantic mackerel where most are landed into Newlyn. Note the shear size of these fish that overshadow their small waterery trawled cousins, usually deriving from the west of Scotland, and caught by Spanish boats.

The best red mullet I have ever had are landed in Newlyn. These fish are really special but unfortunately have suffered, like many species, of becoming fashionable. As restaurants increase their usage the price then begins to rise as many are chasing a few fish.

The cheeky Guarnard is visually my favourite of all the British food fish. The species, Red, Grey and Tub, make up this hugely interesting fish group that has also suffered price wise through its own success

The Turbot, or the king of the sea, is a really special fish. A great dinner party spectacle in its whole form. My fish are mostly from Plymouth, but Newlyn does indeed yield some beauties. A Close relative of the Turbot, and a member of the left eye flatfish family, is the Brill. This is my absolute favourite and fish of this quality are an absolute must.

The John Dory. Simply sublime and a true favourite of our French and Italian customers. Generally served as fillets when eating in a restaurant this fish really must be tried whole as the bones yield so much flavour.

The South West Monkfish is the best in the business. As acceptable as the Scottish monk is it is always a little wet and has much larger flaps that yield wastage. Always look for the nice pink coloured flesh indicating the height of freshness. All the South West ports produce high quality Monkfish and if searched for Monkfish liver can also be located

I defy anyone to find better plaice, haddock, or cod (when available) than that landed at the three South West ports. Always consistently superb quality and a joy to sell on a daily basis. Not much to choose between the ports but it would seem Plymouth yields the best place whereas Newlyn edges the haddock and cod.

My Soles are the best anywhere. The Plymouth Lemons are just amazing and outstrip the Channel, scotch and Icelandic by fathoms. By the nature of the product they are a little more expensive but when reciting the phrase ‘you get what you pay for’, in the world of fish Lemon Sole really matches that theory. I recently spoke about my Dovers in its own blog post to the joy of the West Country.

All hail the West Country as their squid is also the best money can buy. Inshore netting or jigging offers such high quality that a huge percentage of ink sacks are still intact for customers to use in their pasta dishes.

The Scots do some great fish including these live creel caught west coast Langoustine. These are the finest prawn available and if you see these lively chaps you have too try.

Native English Lobsters

These Razors are Scotch and lively little chaps to boot

Sunday, 11 October 2009

Strengths and weaknesses of fish stock assessment techniques

A few years ago my fascination with the internet started when i produced my own fisheries management website. Being a fisheries scientist at heart it was based mainly around capture fisheries and inland fisheries consultancy that at the time was my vocation. I wrote many pieces that are still on the net and viewed by around 70000 people a year. This piece, however, is worthy of a re-load as its relevance is still current. I apologise if it gets a little scientific in places, but please bear with it as it will give a great insight into just how difficult it is to calculate stock levels and issue quotas.

Accurate stock assessment is of growing importance as the human population and demand for fish increases. Continued technological advances in fishing fleets increase efficiency directly effecting natural fish stocks. Attempting to match natural stock fluctuation with fishing effort may help to avoid any further long term damage of exploited species; this is of great importance as fish provide vital contributions to food supplies and influence employment in coastal areas.

Various methods are applied to calculate estimates of recruitment, stock sizes, and age groups. It is apparent that stock assessment techniques are highly dependent on available data, whether long or short-term predictions are the aim, both strengths and weaknesses are influenced by the abundance of this information. For correct predictions many techniques require large inputs of unbiased data, therefore the strength of any stock biomass prediction will be influenced by the weakness of the available inputs; validating final modal estimates of a fishery.

Rose (1997) offers another view for these problems, indicating that fisheries scientists have lost track of their science by becoming 'keyboard ecologists' whom rarely, if ever work directly with real fisheries. This of course does not reflect on the collection of fisheries data but more the interpretation and wisdom required to gain results. This diversion may lead to incorrect long-term analysis; potentially undermining fishery techniques, therefore it is crucial that all stakeholders in a fishery increase an understanding and trust in stock assessment procedures (Anon 1998).

Cortes's (1998) study of shark stock assessment concluded no accurate results could be gained without increased collections of biological and fishery data, coinciding with a better understanding of stock recruitment relationships. Many fisheries are in dire straits due to data collection leaving them with a retrospective problem for stocks. This is pinpointed by Mohn (1999), who studied data on the East Scotian Shelf cod fishery. He concludes that failure to correct the problems encountered by traditional analysis techniques, leads to catch level advice twice or more the intended level. Stock and recruitment data sets should not be published or used unless estimates of error variance are shown; without this information Walters & Ludwig (1981) believe they are meaningless and misleading.

Data types can be split into two groups, dependent or independent of the fishery. Fishery dependent data comprises of four usable types, the total catch, amount of fishing, (the combination known as) catch per unit effort (CPUE) and age or size compositions.

Catch data is essential for most stock production models, inaccurate or biased collection can have damaging long term effects. Importantly, this data should be totalled over ages, fleets, and nations with longer term information helping to predict the past life of the fishery.

Problems arise with data collection as some fleet members find financial rewards in discarding initial catches, searching for larger or higher valued cohorts. When total catches are calculated these discarded fish that obviously made up part of the initial stock are rarely accounted for. The result can be higher biomass predictions, thus allocation of total allowable catches are higher than the available stock. This incorrect estimation of stocks can result in either continued over exploitation or economic hardship when quotas are cut. Some models include discard data, but as Mesnil (1996) points out, the general assumption is that all discards die even though there is proof that in some fisheries (usually shellfish) a significant fraction are able to survive. Either way the incorrect analysis of discarded fish will result in wrong estimates of the fishery.

The 'amount of fishing' or 'effort data' on its own is less important to fishery scientists unlike economists who study activity trends. The collection of days fished at sea and number of boats operating in conjunction with the previously mentioned catch data is much more valuable. This information known as catch per unit effort (CPUE), if based on age and composition, can be a very important factor in fisheries modelling. Effort based production models (PM) use this catch effort data with catches recorded in weight, in an attempt to estimate parameters as a stock production curve. They also assume that effort is closely related to fishing mortality (Kimura et al. 1984).

When writers such as Roff (1983) suggest the catch/effort data is only reliable to detect major fluctuations in population size and "attempts to determine equilibrium yields from catch/effort data are as likely to be successful as finding the pot of gold at the end of the rainbow", doubts over the strengths of models using CPUE data must arise. Collie & Sissenwine (1983) were more liberal in their views but still express difficulties in the standardising of CPUE data from commercial and recreational fisheries. They pinpoint the of 'constant catchability coefficients' and continued 'technical improvements' as the main areas for biased data.

Many models rely on accurate age, length, and age-length calculations of a stock. The difficult nature of collecting this data probably has the most influence hence the most accurate sources are usually from survey vessels. If capable industry collects this data, but usually samples are taken from the catch when landed; the two usual techniques of ageing these fish are via scale and otolith readings. Otolith ageing is less adaptable to a fishery wide sampling program than scale readings, due to the difficult and time-consuming nature of collection. Otolith morphology has also been shown to be an effective tool for stock discrimination in certain species (Freidland and Reddin 1994).

Independent data collection via fishing surveys helps limitations apparent from actual fishery dependent data. As with commercial methods, variable catches and weather conditions affect surveys; technical influences such as mesh size and vessel efficiency may not coincide with actual fleet averages. Changes in vessel efficiency or shifts in effort may not accurately reflect trends of abundance or fishing mortality. Therefore, in the determining of age structures, growth, mortality rates and historical trends, survey techniques may provide the only basis of data collection (Clark 1979). Simplistic assumptions that areas have been swept clean of fish, and common assumptions that trawls are giving unbiased samples both as to species and size of the local demersal fish abundance, may prove inaccurate and damaging in the long-term.

Acoustic methods can be used to either estimate population sizes of pelagic species directly or in conjunction with survey vessels when beam trawling for demersal fish. Engas & Vold Soldal (1992) believe trawl catch rates cannot be relied upon to provide representative estimates and any bias will therefore affect the equivalent acoustic estimates. This may be due to unrealistic requirements such as the confinement of a stock in an area small enough to be surveyed in a set time, at a required intensity, in mid-water not to close to the shore (MacLennan & Forbes 1987). Further assumptions are limited numbers of other species detected, the target strength of the species is accurately known, acceptable weather conditions and no response from the fish to the vessel. Fish densities estimated by horizontal beaming can be up to fifty times higher than vertical beaming due to boat avoidance creating large errors in final data (Kubecka & Wittingerova 1998). Although much interference may be apparent, Pope (1982) still believes the data gained may be valuable when setting precautionary catch levels.

Methods of tagging can be applied to gain imprecise levels of natural mortality (Shepherd 1988), main weaknesses being the sometimes over-expectant assumptions that need to be made. A fixed population with an equal capture rate and no change in catchability level, coinciding with no loss of marks or tags, all seems a little un-realistic. Peterson's closed population method cannot even test these assumptions whereas the Scnabel open population system can, but still with uncertainties. These methods are applied throughout the world indicating that they must work with certain species under perfect conditions. Uncertainties, as with many other techniques do not seem unique, but the methodology of tagging does seem to have greater assumptions than any other method applied to fisheries.

The choice of assessment type will depend on the biology of the species, the time scale required, the area and purpose of the assessment and any specific goal of the fisheries manager. This choice may be difficult as stock production models used for long-term management are frequently no better in the forecasting of the following years CPUE than is the previous years CPUE (Stocker & Hilborn 1981). Long-term assessments aid strategic decisions by managers, information such as maximum sustainable yield (MSY) can be estimated and relationships between stock and recruits can be found. Short-term assessments can reveal information on the likely catch in the next or following years (CPUE), as well as consequences of recruitment in the near future. The latter relates to the suggestion and tactics of long-term strategy. Describing of uncertainties in these strategies is of great importance to managers when weighing the benefits and losses of different techniques. Rosenberg & Restrepo (1994) suggest methods of analysing and assessing risk in management strategies implying that every possible analysis of risk should be undertaken.

Hilborn (1992) pinpoints three dominant approaches to fisheries stock assessment: the investigation into catch at age data, uninvolved models of biomass dynamics, and examination of length-frequency data. He suggests that these methods ignore what is known about the biology of the fish and tend to rely on single types of data. This point is of importance as natural mortality, assumed in many modals may increase via predation or reduced food sources causing large errors in calculations.

Age based methods such as Virtual population analysis (VPA), require catches recorded in numbers at age on an individual cohort basis to solve the exponential form of the catch equations (Kimura et al. 1984). The dependency of knowing the catch at age in numbers is a downfall as age data is costly and technically difficult to obtain. VPA or simpler cohort analysis needs data from various other sources, any of which could be bias or incorrect.

Catch in weight, natural and fishing mortality, weights at cohort, and proportion of mature fish are all required for cohort analysis. Although these methods are the most commonly applied to stock assessment, the large variety of necessary information will have any final say on the weakness of this technique. Interestingly, Agnew et al. (1998) believes that cohorts of certain species have differing dynamics, and therefore should be considered as different stocks. This would render total stock calculation models redundant, with very few other options available to fishery scientists this opinion seems to be alone.

Some typical problems arising with these methods includes the missing of year data, changes in survey techniques and age determination methodologies; Richards et al. (1997) suggests some graphical techniques to portray these uncertainties. The errors encountered in age structure data can to some extent be cancelled by using mean age calculations in the assessment models (Richards & Schnute 1998). These of course are statistical problems that may be lost in complicated calculations. Important and essentially undetectable problems arise with discard levels, the guessing of terminal fishing mortality, and predation mortality (Christensen 1996). The statistical problems can be corrected with the application of more accurate data collection, but these biological influences need highly intensive studies before a complete understanding can be hoped for. The effects of various percentage errors in the population of a year class, due to incorrect values of fishing mortality are shown in figure 1.

Figure 1: This graph plots percentage error of Ni (population of year class at the ith birthday), against cumulative fishing mortality. The under estimation of Ft (fishing mortality at the last age of a year class to which catch data are available) will result in guesses of Nt that are to small, overestimation has the reverse effect. Interestingly, as the cumulative fishing mortality increases errors in both Ft and Ni decrease. If the cumulative fishing mortality is greater than 2 and Ft can be estimated within the given range many users will find errors in Ni and Fi small enough to work with. Accurate estimations of Ni and Fi require careful choices of Ft if the cumulative fishing mortality is small. This case may arise when numbers of recruits to a year class is guessed from catches of partially recruited age groups. Similar graphs allow fishery scientists to produce the error range of their calculations that will aid assessment of their value. Source: Pope (1983)

When age data is sparse or the species cannot easily be aged, length based assessments are an alternative. Chen's (1997) comparison between age and length structured yield-per-recruit models showed length structured techniques better incorporated information observed from fisheries, but age structured gave more precise and conservative estimates of yield-per-recruit. This is the main reason why age structured models are chosen from the conservation perspective in fisheries management. The obvious difference between age and length is that age is a linear measure of time whereas length is non-linear. This makes data interpretations more difficult, more assumptions of growth reductions due to age must be made. Assumptions removed from a model increase accuracy, this is why age methods are preferred if feasible.

A potential strength of fishery science will be the adoption of multi-species models to fisheries that currently utilise single species methods. These models, although essential for future management purposes, seem unreliable and more imprecise than the currently used methods. They require more data that could lead to inaccurate assumptions, thus leaving fisheries in a worse state.

The key area that multi-species models address is predation. It is often assumed that fishing mortality alone is responsible for the variation in fish survival, but in some fisheries, losses to predation can exceed losses to fisheries (Bax 1998). This could indicate that assumptions of natural mortality in single species models are drastically misleading. Mertz & Myers (1997) point out that if bad estimations of natural mortality are used in calculations of cohort strength derived from catch data, the accuracy may be greatly corrupted. Pereiro (1995) supports that where species are not linked to a specific substratum natural mortality will always predominate over fishing mortality thus fishing mortality is not the subsidiary factor. Either way the addition of accurate natural mortality estimations into models must be welcomed.

This review has shown some major problems encountered when estimating populations from a fishery. Strengths seem sparse, maybe the biggest being that these techniques are the only available methods for estimating stock dynamics. Assessment techniques have strengths over each other and it is imperative the correct method is paired to its purpose.

Weaknesses seemed over bearing and many writers have tried to remove errors from previously presented models resulting in a claim that theirs is now the most accurate. Until data collection methods have improved there will always be inaccuracies in results. The addition of computer programs should aid time-consuming calculations allowing scientists to return to the field of study to uncover new methods of improving the currently used stock assessment techniques.

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