ABSTRACT
It has been said that counting fish is like counting trees, except that you can’t see them, and they move. In a sea of uncertainty and variability, fisheries science has the daunting task of providing advice to fisheries managers who are charged with exploiting fisheries for maximum sustainable social and economic benefit. There has been someprogress toward ecosystem-basedmodels of fisheries (see Browman
and Stergiou, 2004, for a perspective), but typically, scientific advice about fisheries is provided tomanagers on a case-by-case basis.Afishery for a particular species is often spatially partitioned into separate management units, particularly if there is little movement of the species fromonemanagementunit to another (asdeterminedby tagging studies, say). These management units are called stocks, and each stock of sufficient importancewill be the subject of a stock assessment. Stock assessments are as varied as the species they assess, but in a nutshell, they seek to predict the consequences of exploiting the fishery under alternative regulations on the harvest. These regulations could include specification of total allowable catch,minimum(and/ormaximum) legal size,durationoffishingseason, areaopen forfishing, size and type of fishing gear, maximum size or horsepower of fishing vessel, and so on. The amount of effort and expense invested in a stock assessment is typically commen-
surate with the perceived social and economic value of the stock, and the will of relevant stakeholders to fund the work. The latter can be particularly problematic for a stock that straddles or traverses geopolitical boundaries. In simpler cases, the stock assessment may utilize only the annual commercial catch rate (Section 23.4.1). At the other extreme, a highvalue stock may be surveyed annually by a dedicated research vessel. A subset of the commercial and/or research catch may be measured for length, and where appropriate also for weight, sex, sexual maturity and age. Age can be determined by counting annual rings deposited in hard body parts – in fish this is typically the otolith (ear bone). Aging is more challenging for crustaceans because they molt their exoskeleton, and also for animals in the tropics because annual growth rings will not be formed if there is little seasonal variability. Larval surveys may also be conducted regularly, by research fishing with a small trawl with a very fine mesh (finer than 1 mm, for example). In addition, further research may be undertaken to investigate other features of the dynamics of the fishery, such as the effects of environmental change and variability, the relationship between recruitment (the number of young fish entering the fishery) and the size of the stock that spawned them (Section 23.4.3), behavior of fish to the fishing gear (Section 23.4.2), impact of recreational fishers, or the amount of (often unaccounted) wastage from the discard of fish that are not of legal or commercially viable size.
