Stock assessment for fishery management - Library

64 Stock assessment for fishery managementNon-equilibrium methodsIn contrast to the methods above, age-based VPAs estimate the fishing mortalityexperienced in each year by each actual cohort or year-class of fish. Standard VPAsestimate the F experienced by each cohort in each year, by comparing the numbersestimated in each group (Z = ln (N t+1 / N t )). This produces a two-dimensional matrixof outputs of both N and F. “Separable” VPA models distinguish the contribution to Farising from the exploitation rate (the variation between years) and the selectivity (thevariation across ages, depending on the mesh sizes, migration patterns etc). Many ofthe comments made above about using VPAs for estimating stock sizes apply equallyto the estimation of fishing mortality.Another possibility for estimating non-equilibrium fishing mortality rates (and stocksizes) is the MULTIFAN-CL software of Fournier, Hampton and Sibert (1998). Thislength-based method provides an integrated approach to estimating age compositions,growth parameters, mortality rates, recruitment, and other parameters, from combinedtime series of fish catches, fishing effort and length frequency data. It incorporatesBayesian parameter estimation, and gives confidence intervals for its outputs. Althoughvery adaptable, the software is not easy to understand and the uncertainties in theoutputs will always likely be higher than for age based methods. Wherever possible,fish should be aged to allow the best analyses.3.4.4 Other indicatorsThe two previous sub-sections have focused on the stock size and the fishing mortalityrate as the two primary indicators of the state of the fish stock and the level of fishingpressure. Most of the reference points that have so far been developed relate to oneor other of these indicators (see following section). Indicators such as these clearlyrequire expensive monitoring programmes and scientific assessments to estimate.They have nevertheless become central elements of the management systems used inEurope, north America and elsewhere (largely based on VPAs and statistical catch atage methods).Of course, many other indicators could also be used. Simpler indicators such asthe percentage of mature fish in the catch (Caddy, 1998) or the CPUE of a standardvessel could be estimated more easily and may be equally acceptable to fishermen asthresholds for management action, especially if they are better understood and seen asmore transparent.As emphasized in Section 2.5.1, a range of goals and objectives should also be setfor the fishery. Ecological objectives should be adopted as limits to exploitation whilesocial and economic objectives may be set as targets to aim at. Indicators and referencepoints should be set for each of these objectives (Section 2.5.2). FAO (1998) suggest tomonitor indicators of fishing operations, biological characteristics, and the economicand socio-cultural objectives of the fishery. They provide 26 pages of examples of thetypes of data that may be collected in deriving such indicators. FAO (1999) furthersuggest that a full “Sustainable Development Reference System” should monitorindicators in each of the ecological, economic, social and institutional / governancedimensions of a fisheries. In their framework, important ecological indicators mayinclude catch rates and catch composition (sizes and species), the state of criticalhabitats, and the fishing pressures in different areas; economic indicators may includethe profitability of different fleets, the value of any fishing rights (e.g. ITQs) andthe subsidies used in the fishery; social indicators may include employment, theconsumption of protein and the maintenance of traditional cultures; and governanceindicators may relate to management capacity, the extent of compliance with fishingrules, and the degree of transparency and participation in the management regime.Readers are referred to the above sources for possible indicators in each of thesecategories.