The starting point for assessing the state of a fish stock is to determine the size of the total catch. Ideally, data must be collected on all the age classes of fish removed from the stocks concerned. Apart from commercial landings, any subsistence catches or fish discarded at sea by fishers in pursuit of other species must be recorded or estimated. Statistics from commercial fisheries are easiest to obtain in the form of the landed weight of fresh, frozen, gutted or filleted fish. These landed weights are converted back to the equivalent of live weights to give the nominal catch. This figure, in turn, is converted to the gross catch by adding estimates of fish lost, consumed or discarded at sea.
The gross catch per se is usually not a good indicator of the size or health of an exploited stock. Variable catches can be obtained from an overfished or an underfished stock, according to the level of fishing. Therefore, catch information is combined with that of the fishing effort required to produce that catch. Unfortunately, fishing effort is not a simple unequivocal quantity or physical unit, making it difficult to define and measure. Theoretically, one unit of fishing effort removes a fixed proportion of the stock, known as the catchability coefficient, jkwhich will vary according to the gear type and the fishing conditions. Therefore, the catch per unit effort (also called catch rate) should provide a direct measure of its abundance, but like many theories, this conclusion is based on assumptions that do not completely hold true in nature.
Effort and catch rate
In practice, fishing effort and catch per unit effort must take account of a wide range of factors known to influence the capacity of a vessel to catch fish. These include the behaviour of the target fish (e.g. shoaling behaviour, dial and seasonal migration, vertical distribution), as well as the characteristics of the vessel (e.g. type, engine power, age, storage capacity), the characteristics of the gear (length or area, mesh size, material, gear-borne instrumentation) and the way it is used (fishing practices), the size and skill of the crew and the use of technical aids (sounders, global positioning systems, helicopters and aeroplanes). Some of these cannot be measured directly and their effects on fishing power and capacity are complex. Nevertheless, it has proved possible to measure the relative fishing power of vessels.
The fishing power can be combined with fishing time to calculate fishing effort, but even here problems exist. For example, the catch by a trawler depends almost entirely on the time that the gear is towed in the water, whereas a purse seiner spends a greater part of its fishing time searching for, approaching and encircling shoals of fish and hauling the catch on board. As a result, the relation between fishing time and fishing power is not simple. Measures of fishing time include: the "soaking time" of a fixed net, pot or longline, the total duration of hauls for a trawler, the searching time for a purse seiner, the number of days at sea or on the fishing ground for any fishing vessel. All of these have a complex relation with fishing power and fishing effort. Nevertheless, with some rough measure of the magnitude of fishing on the stock defined, the final part of the assessment puzzle can be completed: how the impact can be measured and regulated to conserve the fish stock. The fisheries scientist now returns to the basic biological data of age and size by sampling length-frequency distributions from the catch (sometimes supplemented by data from scientific surveys, including information on abundance, size structure, spawning biomass, egg and larvae abundance).
Length-frequency distributions are a first step in determining the numbers and sizes of different ages or year classes in the catch. These measurements, based on samples taken regularly over a number of years, can be used to establish the growth of the fish, the age structure of the population, the age at which the fish become liable to capture, and how quickly the population is reduced as a result of fishing and natural mortality.
Abundance of stock
Fishing reduces the abundance of the stock, causing the catch per unit effort to fall. By the time the population is fully exploited and, if sustainably utilised, at this level losses through fishing and natural mortality can still be balanced by gains through growth and recruitment in the population. Increased fishing also causes a decrease in the average size of fish caught, as the balance in the population shifts from predominantly older, larger fish to younger, smaller ones. The ratios of the various classes in successive years indicate the rate at which the fish die, due to natural reasons (e.g. predation) or due to fishing. Together with independent calculations of natural mortality, this information provides a basis for calculating the population available for capture and the effects of fishing.
Modern approaches forming the basis for multispecies assessments are now taking into account the feeding patterns of, and interactions between target and associated (or dependent) species, in addition to possible interactions between fishing gears and/or fisheries. The ecosystem-based management of fisheries as now being required by most modern fisheries agreement imply an even broader basis for resources assessment including the analysis of the state of the environment, critical habitats, ecosystem variability, climate change, impacts from land-based activities, and impacts on species composition (including trophic chains) and biodiversity (including genetic diversity). This important and developing aspect of fishery resources assessment is, however, in its infancy.