FISHING GEAR SELECTIVITY AND PERFORMANCE

by

Preface

The purpose of this paper is to draw attention to the broad aspects of fishing gear selectivity inasfar as it relates to fisheries management and to the measurement of fishing effort. It is not intended to provide a detailed or scientific analysis of selectivity, rather to give sufficient information for discussion of aspects of fishing gear performance that are of interest to fishery managers.

Selectivity is understood here as the capacity of any method or gear type to capture certain fractions or sections of the fish population whether grouped by species, age, size or behaviour, and to exclude others.

Too often gear selectivity has been viewed only in the very narrow sense of mesh selectivity in the cod-end of trawl nets. This paper takes a much broader view and treats mesh size as a rather minor aspect of total gear selectivity.

Some suggestions are made as to the usefulness and feasibility of management measures based on limitations of fishing gear size type or specification. It is hoped that they, and the general information contained, will be of some help to those involved in the formulation of fisheries legislation.

1. DEVELOPMENT OF FISHING GEAR AND METHODS

1.1 Chief gear types in use 1900–1940

In the early days of fishing and up to World War II, most commercial fishermen had a market for only a few species of fish. As a result of this, the types of gear and the methods they used were extremely selective. They were selective both as to species and as to size of fish taken. Most fishing boats were single-purpose units fishing practically all the time for one species and for fish above a certain size within that species. Some of the common vessels and gear types of the pre-war era are listed below together with the principal species sought.

It was relatively easy to control the fishing effort of the above vessels because many of them were working for one kind of fish and their gear in most cases could not be economically used to catch other species. The nearest thing to a multi-species fishery was bottom trawling, but even there a handful of species dominated the catch, especially in the higher latitudes where the bulk of fishing took place. Fish markets could not cope unexpectedly with less familiar species and there was little incentive to diversify. For most fishermen in the North Atlantic there were only two species that really mattered and these were herring and cod. Under that sort of situation one could fairly easily control the size of fish caught by regulating a minimum mesh size for drift nets and trawls or minimum hook size for long lines.

1.2 Main gear types now in use

The situation regarding fishing effort has altered radically since the war as a result of enormous advances in fishing technology coupled to a considerable increase in and diversification of fish markets. Today, two types of fishing gear account for the bulk of the fish catch in most countries in temperate regions. These two gear types are the trawl net and the purse seine. But within each type there exists a multitude of variations and sub-types which make simple analysis all but impossible. For instance, we could categorize the main trawls and seines as follows:

1.3 Purse-seine use and adaptability

Purse seines tend to be used for a single species of fish and the trawls for a variety of species. There are some purse-seine fisheries where a variety of species are taken, but these are mostly associated with light attraction or the use of fish aggregation devices. Salmon purse seines and tuna purse seines are not really suitable for the capture of other species.

However, although a North Sea purse-seine skipper might use his net chiefly to catch herring and/or mackerel, he might just as easily capture schools of sprat, capelin or pilchard. Provided that the other fish were available and that a market for them existed, this could happen. The fishermen's main concern in switching from one species to another would be that the mesh was small enough (to avoid gilling) and that the net was deep or shallow enough for the grounds and the schools of fish in question.

1.4 Range of species caught in bottom trawls

With trawl nets the situation is more complex. The net types listed have a basic selectivity related to their design and to their use. But they are all used on the sea bed and for the same general group of species. Taking just six of the bottom trawls used in the North Sea, we might illustrate how their use and effect overlap with each other, as follows:

Bottom Trawl Gear	Type of sea-bed on which it is used:
	Rough ground	Hard ground	Smooth ground
Bobbin trawl
Pair trawl
Light trawl
Scottish seine
Danish seine
Nephrops trawl

Although they are all bottom trawls utilized chiefly for demersal fish, the proportion of different species caught varies as a result of the net design, the techniques used and the grounds fished. Note that the catch composition would still be different if all these trawls were used on the same grounds at the same time.

Relative proportions of different species taken by Trawl Nets in the North Sea - 1980

Source: Sea Fisheries Statistical Tables, England & Scotland, HMSO, 1973 & 1980

From the percentages of different species caught by each of the six bottom drag nets, we can see that their relative effectiveness varies considerably. Taking the closest of the above values for each species and the most variant, we can calculate the difference in relative effectiveness of the gear types as follows:

These values reflect the difference in performance during one year only, but are sufficiently diverse to make the point that different types of trawls are selective in different ways. The selectivity of trawl nets is further dealt with in Sections 3.1 and 3.2, and 5.1 to 5.4.

1.5 Midwater trawl use

Midwater trawls are used to catch fish concentrated in schools or in layers anywhere from about 10 metres below the surface to about 10 metres above the sea bed. These concentrations consist mostly of fish of one species, and one size or age group. Herring, mackerel, sprat, anchovy or sardine may be taken by midwater trawls. The fishermen are usually looking for one particular species and they would have a fair idea from the echo-recordings, the place, time, depth and behaviour of the school, what species it consisted of. There is little selectivity in the trawl itself, apart from the size of the bag and codend meshes which are usually small in any case.

Midwater trawls pulled by more powerful vessels may catch faster-swimming, better-quality fish at times than nets towed by smaller boats. But if a layer of fish consists of two species, say herring and mackerel, or herring and pilchard, any midwater trawl towed through them is likely to capture both.

2. TROPICAL AND MULTI-SPECIES FISHERIES

2.1 Range of species and sizes of fish

In a multi-species fishery like the North Sea bottom trawl fishery, three species comprise over 75 percent of the catch. In many tropical fisheries one would count ten or twenty most prevalent species before totalling 75 percent of the total trawl catch.

If it is difficult in the North Atlantic to manage stocks of cod, haddock and whiting mingled together and caught together, how much more complex is it to manage three or four times as many species. This is a major dilemma facing fishery managers in the warm water countries.

In most developing countries all fish caught can be marketed, whether large or small, mature or immature. There is little incentive for any fisherman to throw juvenile fish back into the sea. The range in size and shape of fishes caught is such that any mesh restriction to limit capture of immature fisih of one species is bound to result in the loss of many mature fish of another species.

2.2 Selective types of gear

However, of fishing gear types and methods used in the tropics, some are extremely species selective. Even in waters where other species abound, their use does not create a complex management problem. The main selective gear types are listed below.

2.3 Less selective gear types

Fishing gear and methods which are less selective include most of the demersal fish trawls, the seines and lift nets used in conjunction with light attraction or FADs, and many of the artisanal traps, lines and tangle nets. They are listed below.

* Used with light attraction or FADs

3. GEAR SELECTIVITY

3.1 Selectivity of Trawls by Design

Trawl nets and bottom seines possess an initial selectiveness owing to their particular design and mode of operation. A mesh size limitation would vary considerably in effect from one type of trawl net to another. It is important therefore to consider gear selectivity and performance before we begin to think of mesh sizes and their effect.

Take a herring vinge trawl for example and two other bottom other trawls like the granton trawl and the shrimp trawl. All use otter boards, all are on the sea bed throughout the duration of the tow and all have the same basic structure of wings, square, bellies, bag and cod end. But their performances differ as much as those of a racing car, a truck and a tractor. All three nets could be fishing in the same area, yet the vinge trawl might take only herring, the granton trawl only demersal fish and the shrimp trawl primarily shrimp or prawn. Mesh size has little to do with this species selection. It is a function of other aspects of gear performance - speed of tow, headline height, ground contact, flow of water through the body of the net, otter door spread, length of ground-cables, and so on. The vinge trawl would have the smallest mesh in the cod end, yet it would take no cod, haddock or shrimp. The shrimp or prawn trawl would have a smaller mesh than the granton trawl yet it would capture only a small proportion of the bigger fish the granton trawl would take. To imagine that in these cases the size of mesh controls the size of fish caught would be as simplistic as to think that the respective speeds of the racing car, truck and tractor were determined by the size of their wheels.

In contrast to the trawls designed to capture one species or group of species, there are the multi-species trawls or combination trawls which take a great variety of fish. The North Sea prawn and fish trawl is a combination net designed to capture Nephrops norvegicus or Norway prawns, plus demersal fish like cod, haddock, skate, monks, plaice and lemon sole. Most bottom trawls in the tropics and sub-tropics are multi-species trawls taking fish which vary greatly from each other in size and shape. There is no common selectivity factor and no common minimum size or length for the various species. For these fisheries, a mesh regulation is at best an inadequate means of reducing juvenile mortality. It can be aimed only at the smallest of the main commercial species.

3.2 Selectivity by area swept

Many types of bottom gear depend on the area of sea-bed swept for their effectiveness in fish capture. Ropes, wires, cables and bridles are used to herd fish in the direction of the net. It would be uneconomic in terms of gear and power necessary to sweep such large areas with walls of netting.

Several species of fish respond to herding by ropes, and possibly just as many do not. Generally speaking, the demersal fishes of temperate waters are vulnerable to this type of gear while crustaceans and most pelagic fishes are not. There are several technical, environmental and behavioural reasons for this. Fish respond, not just to the approach of ropes and cables but also to the noise or vibration of the otter boards, and to the clouds of mud the boards leave behind. When the angle and the speed are such that the expanding trail of mud-cloud just touches the ground cable or sweep lines, then for many demersal fishes the herding effect is maximized; this applies only to trawls utilizing otter boards. Danish seiners and pair trawlers are able to herd fish effectively without disturbing the sea bed unduly.

The species that do not respond to herding by ropes include practically all of the shrimps and prawns, and most of the clupeids and mackerels. Trawl nets designed to capture shrimp have a wide but low net mouth and extremely short cables or bridles. Trawl nets for clupeids and mackerels have very large net mouth openings. To make the net mouth as big as possible without creating too heavy a towing load, large meshes are used. These meshes are big enough for the fish to swim through with ease, but this does not happen if the net is properly constructed. So pelagic species while they do not respond much to ropes or wires, will be guided by sheets of tight netting even if the mesh sizes are very large.

Examples of gear types which depend primarily on sweeping a wide area of the sea bed are listed below:

Bottom seines
(beach seines, Danish seines, Canadian pair seines)

Pair trawls
(bottom pair trawls, utilizing long warps and cables)

Otter trawls with Vigneron Dahl gear
(long cables inserted between the otter boards and the net or bridles)

Fishing gear which does not rely on herding fish from a wide area on each side of the trawl net is mainly used to catch shellfish. Some common examples are listed below:

Midwater trawls are used almost exclusively on schooling fish or scattered layers of schooling fish, and skippers always aim to tow the net directly into or through a school or layer. There may be some scaring effect from otter boards, weights, bridles or kites, but it can hardly be considered a herding effect.

3.3 Effect of tickler chains and ground ropes

The ground rope of a trawl net has an effect on the amount or proportion of flatfish and shellfish that may be caught. A heavy groundrope will tend to scrape tightly on the sea bed while a lighter one may travel more gently over the ground. The degree of abrasion on the sea bed can also be controlled to some extent by the slackness of the lower wings and by adjustment of the lower bridle chains.

Tickler chains are often used to dig up the sand just ahead of the ground rope. This drives flatfish and some shrimps and scallops up into the mouth of the net. Vessels working for Dover soles may use several chains, each slightly shorter than the other and thus dragging a foot or so ahead of each other in front of the net. Worn chains or chains of softer steel or alloy are not so effective.

Beam trawls fitted with a series of tickler chains are extremely effective in capturing sole which other trawl nets might scarcely touch. This may not be a bad thing in itself as the sole which bury themselves in the sand might not otherwise be harvested in quantity. What may merit concern is the possible damage to other marine life on the sea bed, from the disturbance caused by this heavy gear.

3.4 Purse seines and ring nets

Purse seines and ring nets or lamparas are selective both by their use and their design. Most large purse seiners operate chiefly on one species like tuna, anchovy or salmon, or on two or three species like herring, mackerel, pilchard and sprat.

With purse seines, one is dealing with schooling fish of one species or with a body of mixed fish attracted by light or FAD. Most purse seine skippers know what species of fish they are setting on, at least 90 percent of the time, from the nature of the echo-traces, the locality, time of year, and the behaviour of the school. Exceptions to this are the smaller light-fishing purse seiners in the tropics or sub-tropics. They take a very mixed catch of sardines, anchovy, horse mackerel, bait fish and some larger species like tuna and mackerel. Occasionally a lot of very young tuna are taken.

It has sometimes happened that a North Atlantic seiner, searching for herring or mackerel, has netted a school of whiting or cod. But this occurs so rarely and in such unusual circumstances that it does not present a management problem.

Mesh sizes are important to a seiner skipper mainly because he wants to avoid gilling fish whatever happens. A mesh that is too small poses a very minor problem compared with one that is not small enough. If a catch of very small fish is taken (and schools tend to be made up of fish of one size) then it is a simple matter to let them go while they are still alive. There is much more likelihood of fish surviving if released from a purse seine than from a trawl net after it has been hauled up. Both the density of fish in the bag, and the sudden pressure decrease from the seabed to the surface, can cause mortality before the cod end is released.

Purse seines in Lake Kinneret carry a filter panel at the top of the bunt to permit the escape of small fish. The filter panel mesh is 20 to 25 percent larger than the other seine meshes. Small fish are allowed to swim through the panel before the catch is dried up in the bunt. Some fish are gilled but they can be shaken out of the filter netting. This could hardly be done if the larger mesh was used throughout the body of the net.

An interesting example of how a purse seine can be made to be species-selective is seen in the United States tuna fishery. Schools of dolphin or porpoise were sometimes netted along with tuna and there was considerable pressure from conservationist and environmentalist groups to prevent unnecessary slaughter of these animals. The solution adopted involved a “backing” operation and the use of purse seines fitted with shallow escape lips for the porpoise schools. With skill and practice it became possible to release the dolphins alive without losing any of the valuable tuna catch.

3.5 Selectivity of gill-nets

Gillnets are obviously size selective and will seldom capture fish small enough to swim through the meshes. Fish too large to be gilled properly will sometimes be entangled in the netting but this is not always so.

In the early days of the Lake Kariba fishery, some of the indigenous Zambesi fishes grew to remarkable sizes. Tilapia mossambica was one of these and being highly marketable it was much sought after. In 1961 the gillnets used were of 4 and 5 inch mesh chiefly as these were the sizes used in Lakes Mweru and Bwangwelu. During 1962 and 1963 nets of 5½ and 6 inch mesh were successfully introduced and these caught large tilapia which the other nets rarely took. Nets of 6½ and 7 inch mesh were then tried out and even larger fish taken, but their numbers were much smaller. Experimental nets of 7½ and 8 inch mesh were also tested but these caught no fish at all. The experience is mentioned to indicate how large fish can avoid capture by a gillnet. Fishermen can get over this to some extent by hanging their nets very loosely.

Another rarely mentioned aspect of gillnet selectivity is that inactive fish are seldom caught. Just as a long line will only hook hungry, feeding fish, a gillnet will only capture fish that attempt to swim through it. During cold weather or in the dry season, fish in most African lakes become very inactive. Their metabolism slows down to the point where they hardly move or eat. During such periods they will rarely be caught by a gillnet, unless they are driven in as happens in some fisheries. Fishermen in northern Newfoundland and Labrador have observed cod behaving in this way in small bays or creeks, at certain times of the year.

In temperate waters, gillnets tend to be used to capture a single species - herring, cod, salmon or plaice. In the tropics a multitude of species are caught. Thus a mesh restriction to protect juveniles works well in temperate fisheries or in a single species fisheries but is rather pointless elsewhere.

Since the decline of drift netting for herring, the main gillnet fisheries in temperate waters, have been those for salmon and for cod. The bulk of gillnet fishing in the world today occurs among the small-scale fisheries of Asia and Africa. There, such a wide range of species and sizes is taken that control by mesh restriction is out of the question in all but a very few isolated cases.

4. SELECTIVITY OF FISH ATTRACTION METHODS

The three main methods of fish attraction are by light, by bait, and by floating rafts and artificial reefs. Each method is selective under different conditions. The selectivity is the product of the combined effect of the attractant, the fishing gear, the location and the method or application. Note also that secondary attraction occurs. Larger species come to prey on small fish congregating around the attractant.

4.1 Light attraction

Lights are used throughout the tropics and sub-tropics to attract fish for capture by purse seine, ring net or lift net. Light attraction is not much used in temperate waters owing to negative or unpredictable reaction by fish. In many cases there juveniles are attracted and mature fish repelled or left unaffected.

Most of the small pelagics in the tropics display positive phototaxis. These include sardinella, anchovy, saury, small mackerel and bait fish. Squid are also attracted to light but prefer to stay in the fringes of the illuminated area. Larger fish are attracted by the concentration of small fish, and by their movement or increased activity in the presence of light.

Thus catches made with light attraction and seines or lift nets tend to be of mixed species of fish. Exceptions to this are squid which are taken by special jigs that would not catch other species, and surface swimming saury which are taken by stick-held dip nets in waters north of Japan.

Catch composition may differ from area to area. The same net and lights may catch different fish if employed over shallower or deeper waters.

In Lake Tanganyika, dagaa and ndakala sardines are taken along with perch and Nile perch. The dagaa of Lake Kariba are accompanied often in light-attracted catches by quantities of Alestes species. In S.E. Asia, sardinella, mackerel, scad, anchovy and tunas are taken together. Light-induced catches in the Mediterranean may consist of mackerel, pilchard, sardinella, anchovy, sprat, immature barracuda and occasional tuna.

Small light-sticks are now used on certain baited long lines, drop lines and traps. They are used on swordfish long lines and on hand lines and fish traps for large demersal species in places like the Caribbean.

4.2 Bait attraction

Bait is possibly the oldest means of attracting fish and it is still in widespread use today, particularly in traps and pots used for lobster, crabs, prawns and rockfishes. Longlining for cod, halibut, ling or dogfish in northern waters is coming back into use after a decline of some years. Tuna long lining is still practiced in the Pacific and Indian Oceans. Swordfish long lines are used off the eastern seabord of the USA. Small-scale fishermen in some parts of the world use bottom-set long lines.

Most of these methods are fairly species selective. While one still gets crabs in lobster pots, skate on cod lines, and shark on tuna lines, they are the exception rather than the rule. The baited lines and traps can be considered to be selective in both attraction and capture. This is partly the case, of course, because they are set in areas and at depths where the fish sought are known to be present.

Live bait fish are used to attract schools of skipjack, bonito and other tuna for capture by pole and line. This is a very selective method, since the schools are detected visually first and since the fishermen can see what they are taking.

Some of the live bait used is itself caught by bait - a ground meal or powder made of cereal, fish or other protein. The use of this kind of bait is usually called “chumming” and it is normally selective in attracting only the small bait fishes needed by the pole and line vessels.

4.3 FADs and artificial reefs

Fish aggregation devices, or FADs for short, are used to attract pelagic fishes in the tropics. Tuna purse seiners and pole-and-line vessels fish around those anchored in deep waters off the islands of the Indo-Pacific. Smaller purse seiners and lift-net boats fish around FADs anchored in shallower waters nearer land where sardines and small mackerels may be found.

The FAD itself is a floating raft to which are attached strings of palm leaves or old ropes, netting or tyres. The attraction of fish to these devices is not fully understood. It may be partly for shelter or protection, partly for orientation, and partly to feed on the smaller fish or algae that accumulate there.

As with light attraction, a mixed body of fish is attracted to a FAD. Light or bait attraction is often used to supplement the drawing power of the FAD, or to draw fish in closer before capture. The small fish usually swim closest to the raft, and the larger ones further away. Big tuna may be caught by hand line many fathoms below a FAD.

Artificial reefs, as their name implies, attract mainly reef fish and crustaceans. They are installed on bare stretches of the sea bed where the natural habitat is poor. The artificial structure becomes a favourable habitat for many of these species. Although few of them have been constructed in northern Europe, fishermen there for many years have practised fishing close to rocks or wrecks where fish tend to congregate.

The selectiveness of fishing methods used in conjunction with attractants may be summarized as follows:

5. EFFECTIVENESS OF MESH REGULATIONS

Minimum mesh sizes have been introduced in most well-developed trawl fisheries to reduce the fishing mortality of undersized or juvenile fishes. That small fish can escape through cod-end and bag meshes has been confirmed by numerous underwater films and observations. Whether the mesh regulation has a significant impact on the number of juveniles surviving to maturity is not always certain. Other factors must be examined, and these include the post-release mortality rate, the percentage of juveniles that would be caught in trawls, vis-a-vis the total juvenile population, and periodic natural fluctuations in the stock of fish in question. An important factor in fisheries where adjoining countries harvest a common stock of fish, is the existence and enforcement of similar legislation in each country concerned. There is little incentive for one state to protect young fish if its neighbour is harvesting them regardless.

As trawl nets differ greatly in performance, we will examine the effects of minimum mesh sizes on three main types of bottom trawl: multi-species bottom trawls; bottom trawls for pelagic species; and shrimp trawls. Note that in each case the minimum mesh size applies to the bag and cod end only. The meshes in the wings, square and shoulders are much larger.

5.1 Bottom trawls for demersal species

These trawls are used all over the world, chiefly by inshore or nearwater vessels, but the variety of species caught tends to increase towards warmer waters. A further complication in the tropics is that very small fish are quite saleable on local markets. In temperate countries consumers tend to avoid small fish, so if these are caught they generally go for reduction to meal, oil or pet food.

In a temperate fishery most fishermen would accept a mesh restriction without much argument. There are two situations in which it might be resented. One concerns fish which owing to their shape tend to escape through meshes which would stop mature fish of a similar species. The whiting, Gadus merlangus, is an example of this. Though it resembles the haddock, Gadus aeglefinus, a mesh that would retain mature haddock would permit fairly large whiting to escape. Conversely, a mesh that would retain mature whiting might also retain immature haddock. Further legislation introduced to cope with this problem has involved minimum sizes for fish sold for human consumption. However, that does nothing to prevent numbers of small haddock being killed in order to capture mature whiting. In that kind of situation, if the mesh regulation aimed at haddock is obeyed, large quantities of mature whiting will not be caught. If it is infringed, or area by-laws introduced to permit smaller meshes, some immature haddock and other fishes will be destroyed.

The other situation concerns fisheries where the income from industrial fish (fish sold for conversion to meal, oil or pet food) is a significant portion of the gross earnings. This creates an incentive to keep as large a by-catch as possible. If the by-catch is pout, sand eels or sprat, little harm is done, but if it is young herring, whiting or haddock, then other fisheries could suffer. Legislation to limit the harmful effects of industrial fishing usually relates to the proportion of juveniles of other species in the by-catch. However, unless there is strict inspection, infringements will occur regularly, and this makes enforcement both troublesome and expensive.

In tropical and Mediterranean fisheries, bottom trawlers tend to keep all fish caught for human consumption, regardless of size. This results in the landing of many juveniles and the constant incentive to use small-meshed cod ends. A minimum mesh size regulation is thus difficult to enforce and if enforced it could result in the loss of many mature edible fishes like squid, lizardfish and wolf-herring. Further complications exist owing to the pelagic and semi-pelagic behaviour of many of the species. They can be trapped in seines or lift nets which do not have a mesh restriction. So immature fish escaping from a bottom trawl in the tropics may still be caught in another type of gear before they reach maturity.

5.2 Bottom trawls for pelagic species

These are mostly herring trawls and tend to be Scandinavian vinge trawls or high-opening semi-pelagic nets of the Polish or German type. Since they are used to catch herring, mackerel, sardine, sprat or sand-eels, they must of necessity use small-meshed bags and cod ends. When properly rigged and operating for the prescribed species these trawls take very little other fish of any description. However, it is possible to trim the gear in such a way that it will take demersal fish in quantity. This can happen when there are no herring or sprat around and the skipper wishes to make up a load of any kind of fish. However, as the mesh size remains the same, many immature white fish could be taken. These would either be dumped (dead) at sea or brought in for industrial use. The point is that once trawlers are permitted to use small-meshed bottom trawls for pelagic fish it is almost impossible to prevent their use on demersal stocks if the pelagic species are scarce.

5.3 Shrimp trawls

Three catch problems associated with shrimp trawls concern the by-catch of mature edible fish which is usually discarded, the capture and destruction of immature fish and the capture of juvenile shrimp.

The fish by-catch problem cannot be tackled by mesh size limitations, otherwise no shrimp would be caught. Separator shrimp trawls have recently been designed in an attempt to deal with the problem. In these trawls, much of the by-catch is separated from the shrimp and released before it reaches the cod end. But this necessarily involves some loss in shrimp catch, additional gear expense, and the inconvenience of handling and repairing a more complex net.

An alternative solution adopted by some states is to insist that the shrimp trawlers retain all the mature fish, or a significant proportion, and bring them back to port for sale on local fish markets.

Only the smallest juvenile fish will escape through a mesh size designed to retain shrimp. Whether the destruction of young fish by shrimp trawls is sufficient to merit concern is debatable. Many young fish are high swimming and shrimp trawls have low headline heights so the juvenile catch may not be excessive, but this could be determined only after proper investigations.

Many shrimp fisheries do not have even a minimum mesh size to protect juvenile shrimp. In tropical countries where small shrimp can easily be sold for human consumption and for making shrimp paste, there is not much motivation among fishermen to protect juveniles. Surprisingly, several such fisheries seem to have survived many years of intensive fishing with small-meshed trawls. The Malacca Strait fishery is an example of this.

A mesh restriction for shrimp trawls to protect juvenile shrimp would appear to be in the interests of all concerned, and would have few undesirable side effects. But enforcement would have to be strict throughout the whole area. The fish by-catch problem could not be tackled by mesh regulation, only by use of separator trawls, or by insisting that mature fish be preserved and taken back to port for sale. Unfortunately, both these solutions may be costly or difficult to enforce.

5.4 Other observations

Cushing (1974) states that “conservation by mesh regulation is least conservation because it is adapted to the smaller and numerous species like the haddock in the North Sea; larger species (for example, cod or turbot) are not necessarily conserved there as well as they might be.”

When considering the effect of a mesh size one must bear in mind not only its dimensions but its shape when the net is being towed. The twine material also has an effect as has the accumulated weight and volume of fish in the cod end at any one time.

A further point about mesh size regulations concerns the ability of resourceful fishermen to get around them. Let us take a hypothetical case of a bottom trawler working for demersal fish in the North Sea. In his area the minimum mesh size is say 80 mm. This is fine for haddock and cod, but suppose the vessel encounters quantities of whiting, most of which can escape through an 80 mm mesh. What can the skipper do if he wants to get around the regulation? He could (a) change the cod end temporarily; (b) add a blinder or second bag and cod end over the first; (c) restrict the meshes in some other way so they do not open fully. Unless he was apprehended in the act of fishing with either modification, it would be extremely difficult after the event to prove any infringement of regulations. There are in fact ways of accomplishing (c) which would probably not represent a breach of regulations at all, e.g. doubling the number of meshes in the cod-end circumference.

6. SUMMARY AND CONCLUSIONS

This paper was prepared primarily to outline the existing situation and to draw attention to problems rather than solutions. Some lessons appear to be obvious however and these are mentioned in conclusion.

The complexities of modern fishing-gear selectivity and performance are such that to attempt to limit the less desirable effects of fishing by legislating on the design, size or specification, of the gear used would appear in most cases to be scarcely worth the effort. Only after extensive and thorough studies can the full or precise effect of any modification be determined.

Nevertheless in isolated, well-studied cases, an enforceable mesh regulation may be a useful component of an overall management system. A size limitation on the depth and length of purse seines, pound nets or gillnets may also be useful in controlling effort in specific fisheries.

In other situations it may be more feasible and sensible to impose a complete ban on certain types of gear in given areas, or a limitation on the number and size of fishing vessels. If a size limitation is to be placed on fishing boats it would be much more practical and meaningful to make it a limitation on engine power rather than length or tonnage as has been often done in the past.

Any limitation or regulation by power must also be related to method of fishing, particularly active methods such as seining and trawling. Engine power has much less effect in vessels using passive systems like gillnetting and long lining.

BIBLIOGRAPHY

Much of the information touched on in this paper can only be verified by verbal communications with fishermen. Documentation on gear selectivity and performance tends to focus on the effect of mesh size limitations. There are now a number of useful films of fish behaviour, vis-a vis fishing gear, and these are available at gear laboratories in North America, Europe and Japan.

Ben-Tuvia, A. and W. Dickson (eds), 1969 Proceedings of the Conference on fish behaviour in relation to fishing techniques and tactics. Bergen, Norway, 19–27 October 1967. Report. FAO Fish.Rep., (62)Vol.1:47 p. Issued also in French and Spanish.

Ben-Tuvia, A., 1969 Proceedings of the FAO Conference on fish behaviour in relation to fishing techniques and tactics. Bergen, Norway, 19–27 October 1969. Vol.2. Review and experience papers. Actes de la conférence FAO sur le comportement du poisson en fonction des techniques et tactiques de pêche. Bergen, Norvège, 19–27 octobre 1967. Vol.2. Etudes spécialisées et comptes rendus d'expériences. Actas de la conferencia de la FAO sobre el comportamiento de los peces en relacion con las técnicas y tacticas pesqueras. Bergen, Noruega, 19–27 octubre 1967. Vol.2. Documentos de informacion y experiencias. FAO Fish.Rep./FAO Rapp.Pêches/FAO Inf.Pesca, (62)Vol.2:49–461

Ben-Tuvia, A., 1969 Proceedings of the FAO Conference on fish behaviour in relation to fishing techniques and tactics. Bergen, Norway, 19–27 October 1967. Vol.3. Experience papers. Actes de la conférence FAO sur le comportement du poisson en fonction des techniques et tactiques de pêche. Bergen, Norvège, 19–27 octobre 1967. Vol.3. Comptes rendus d'expériences. Actas de la conferencia de la FAO sobre el comportamiento de los peces en relacion con las técnicas y tacticas pesqueras. Bergen, Noruega, 19–27 octubre 1967. Vol.3. Documentos de experiencias. FAO Fish.Rep./FAO Rapp.Pêches/FAO Inf.Pesca, (62)Vol.3:463–884

Ben-Yami, M., 1976 Fishing with light. Farnham, Surrey, Fishing News Books for FAO, FAO Fishing Manuals, 121 p.

Caddy, J.F., 1979 Some considerations underlying definitions of catchability and fishing effort in shellfish fisheries, and their relevance for stock assessment purposes. Manuscr.Rep.Fish.Res. Board Can.Halifax, (1489):18 p.

Fishing News International and Fishing News (eds), 1964 Modern fishing gear of the world: 2. West Byfleet, Surrey, Fishing News (Books) Ltd., by arrangement with FAO, 603 p.

Graham, M., 1948 Rational fishing of cod in the North Sea. London, Edward Arnold, 111 p.

Innes, L., 1974–82 Numerous papers and communications. Fraserburgh, Scotland, I.C. Trawl,

Jones, R., 1976 Mesh regulation in the demersal fisheries of the South China Sea. Manila, South China Sea Fisheries Development and Coordinating Programme, SCS/76/WP/34:75 p.

Kristjonsson, H. (ed.), 1959 Modern fishing gear of the world. London, Fishing News (Books) Ltd., for FAO, 607 p.

Kristjonsson, H. (ed.), 1971 Modern fishing gear of the world: 3. Fish finding, purse seining, aimed trawling. London, Fishing News (Books) Ltd., for FAO, 516 p.

Nomura, M. and T. Yamazaki (comps), 1975 Fishing techniques. Tokyo, Japan International Cooperation Agency, 206 p.

Pope, J.A., 1966 Selectivity of fishing gear. FAO Fish.Tech.Pap., (41):41 p. Rev.ed.publ. 1975