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Stock assessment entails more than just the measurement of catch
Stock assessment entails more than just the measurement of catch
FAO/17790/A.Conti

Background

The assessment of the size and state of the stocks exploited by fisheries is one of the pillars of modern management. Direct observation of fish populations over a wide area is impossible without specialised remote sensing electronic instruments (such as echo sounders and echo integrators used in acoustic surveys). Fisheries scientists have also developed techniques to estimate the nature, abundance, distribution, structure and population dynamics of fishery resources from catch data.

In most fully developed fisheries, a sizeable part of the fishable stock, often close to half of it, may be available for sampling, identification and measurement at the place of landing.  This may not be possible when the catch is processed on board (e.g. on factory ships) and is particularly difficult for dispersed small-scale fisheries. When the age of fish can be easily determined, e.g. from the fish hard parts (e.g. otholiths or vertebrae) or analysis of size frequency, growth and death rates (resulting from both fishing and natural causes) can be readily estimated and fed into models. When this is not possible, tagging methods can be used to obtain the same result plus information on migration).

Assessing stocks

Stocks are assessed using two alternative approaches (sometimes simultaneously), depending on data available. Synthetic methods use the theoretical relationship between the level of fishing intensity and the total catch. Analytical methods use the relation between recruitment, growth and mortality.

With both approaches, 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 ought to 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.

Fishing reduces the abundance of the virgin stock, causing the catch per unit effort to fall. It also reduces the lifespan and the average size and age in the population, increasing however its biological productivity (production per unit of biomass). According to the synthetic models theory, for a large range of fishing levels, losses through fishing and natural mortality can be balanced by gains through growth and recruitment in the population, maintaining the population in equilibrium. At some intermediate level of fishing, the population is “ fully exploited” , i.e. exploited at the maximum level of productivity.

Effort, catch rate and abundance

The gross catch per se is usually not a good indicator of the size or health of an exploited stock as it depends both on stock size and fishing intensity. Therefore, catch information must be combined with that of its corresponding fishing effort to generate a catch per unit of effort (or catch rate)..Unfortunately, fishing effort is not a simple variable as the vessel effectiveness in catching fish depends on 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).

Standardizing fishing effort

Some of these characteristics cannot be measured directly and their effects on fishing power and fishing capacity are complex. Nevertheless, it has proved possible to compare and standardize the relative fishing power of different vessels (e.g. using vessel length, engine power, or other relevant indicator as an index of power), combining it with fishing time to calculate standardized fishing effort, with some problems.

Scientist assesses fish stock
Scientist assesses fish stock
Courtesy of NOAA

For example, for a given stock size, the catch by a trawler depends almost entirely on the time that the gear is towed on the bottom, The catch of a purse seiner, however, fishing on visible surface shoals, may depend more on the time and effectiveness of its searching, detecting, and approaching a shoa, than on the time it takes for encircling and hauling it on board. As a result, the relation between vessels characteristics, fishing time and fishing power is not simple. While the number of days at sea or on the fishing ground is an indication of fishing effort for any fishing vessel, measures of effective fishing time will be “soaking time” for vessels using fixed nets, pots or longlines, total duration of hauls for trawlers, searching time for purse seiners, etc. All of these have a complex relation with fishing power and fishing effort.

As a consequence, fishing effort needs to be standardized (across vessels and fleets) so that one unit of standard fishing effort removes a fixed proportion of the stock, i.e. generates a fixed and known amount of fishing mortality. The parameter that relates standard fishing effort and fishing mortality is known as the catchability coefficient. After standardization, the catch per unitof  effort (also called catch rate) provide an indirect measure of stock abundance. Because of the many assumptions needed in the computations, this measure is affected by statistical variance, and possibly by bias, that need to be accounted for in the stock assessment process.

Nevertheless, with a standardized measure of the magnitude of fishing on the stock, the final part of the assessment puzzle can be completed using a synthetic (production) model to establish the trend in the stock and the fishery, their present state, the stock potential, and the catch and effort regulations needed to maintain the stock at a high level of productivity.

Analytical methods

Length-frequency distributions are a first step in determining the numbers and sizes of different ages or year classes in the catch that are needed for an analytical assessment of a stock. These measurements, based on samples taken regularly over a number of years, can be used to establish the age structure of the population, the growth of the fish, 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. Samples of the catch are obtained on landing sites and are sometimes supplemented by information from scientific surveys concerning abundance, size and age structure, spawning biomass, egg and larvae abundance). The ratios of the numbers in 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. It is then possible to proceed to using analytical models to reach similar conclusions regarding catch and effort regulations as well as other interventions regarding mesh size, minimum landing size, closed seasons, etc.

Modern approaches

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). In addition, the growing requirement for a more systemic approach to fisheries assessment, including an assessment of its natural, human and governance components, call for more integrated processes of assessment and elaboration of advice, with a more active participation of stakeholders, and the collaboration of many more disciplines that those involved in conventional management.

These important developments of fishery resources assessment are just taking place and are still far from being fully operational in most countries.

 
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