Exploited marine invertebrates: genetics and fisheries

The application of genetic techniques to invertebrate fisheries is in many ways essentially similar to that in vertebrate (i.e. finfish) fisheries, fo r which there is already an extensive body of published data. However, ther e are also relative differences which lead to particular problems in the us e of genetic data to study commercially important invertebrate species. The main role for genetics of both vertebrates and invertebrates has been, and is likely to continue to be, the identification of groups of interbreeding individuals as the basis for a fishery. It is in the identification of the breeding unit that the genetic differences between vertebrates and inverte brates can be of practical significance. The genetic breeding unit, usually called a 'stock' in fisheries biology, generally shows a certain uniformit y of size in most marine fish which have been studied. Smaller or less mobi le fish (e.g. flatfish) may only range a few tens of kilometres to their br eeding grounds, whilst in more mobile, particularly migratory pelagic speci es (e.g. Scombridae), the area occupied by a stock is likely to be far grea ter and for a few (e.g. large pelagic elasmobranchs), a single unit of stoc k may be almost circumglobal. However, marine fish generally, particularly those large or plentiful enough to be of commercial interest, are likely to be fairly mobile and in many cases the order of mobility is likely to be i n the region we might predict from our knowledge of the biology and habits of the species. In the genetic assessment of 'stocks' for invertebrate fish eries, we face a number of additional problems, mostly related to the large evolutionary range of invertebrates exploited and their widely different b iology. Although in Europe and North America marine invertebrate fisheries may be thought of as being mainly for decapod crustaceans and bivalve mollu scs, globally commercially important marine invertebrate fisheries range fr om sponges to squid and include such diverse groups as sea cucumbers, barna cles, krill, octopuses, cuttlefish, sea anemones, ascidians, polychaetes, s ea urchins, gastropods and jellyfish. An obvious feature of many of these i nvertebrates is that the adult (i.e. commercial) stage of the life cycle is sessile (e.g. barnacles, sponges, ascidians) or of very limited mobility ( e.g. sea anemones, sea urchins, bivalves, gastropods), with the result that the dispersive phase of the life cycle is the larva. Other groups (e.g. kr ill, jellyfish) are planktonic or nektonic and may cover very large distanc es, but, unlike fish, have little control over the distance or direction of travel, whilst some of the open ocean pelagic squid are more mobile than m ost fish and may migrate thousands or kilometres to spawning grounds. The v ery low mobility of both larva and adult in some invertebrates indicates th at dispersal, and hence stock size, is likely to be low and that, therefore , stocks are far more vulnerable to overfishing than in most fish species. An additional difficulty is that genetic studies to date indicate a remarka bly high incidence of cryptic speciation in marine invertebrates, sometimes even in comparatively well studied commercially important species. Thus, a lthough to date marine invertebrate fisheries have not received the same le vel of attention from geneticist as finfish fisheries, it is clear that for invertebrate fisheries genetic data are relatively far more important if a fishery is to be exploited without being endangered.