Geoffrey P. Kirkwood

Stock assessment and the provision of management advice for the short fin squid fishery in Falkland Islands waters

Abstract The scientific basis for managing the short fin squid, Illex argentinus , Stock around the Falkland Islands is presented. A target proportional escapement policy is used which permits a level of fishing effort compatible with conservation targets to be set each year. This policy is intimately related to the method of assessing stock size and the rate of fishing mortality presented in a related paper. The results of applying this management procedure for the 1987 and 1988 fishing seasons, the first 2 years of regulated fishing in Falkland Islands waters, are described. The policy has the considerable benefit that the data requirements and monitoring procedures are straightforward, and can be implemented by the limited manpower resources of the islands.

The estimation of potential yield and stock status using life–history parameters

Using life–history invariants, this paper develops techniques that allow the estimation of maximum sustainable yield and the fishing mortality rate that produces the maximum yield from estimates of the growth parameters, the length at first capture and the steepness of the stock recruitment relationship. This allows sustainable yields and fishing capacity to be estimated from sparse data, such as those available for developing country fisheries.

The utility of otolith weight as a predictor of age in the emperor Lethrinus mahsena and other tropical fish species

Abstract This study examines the potential use of otolith weight as a proxy for age in the lethrinid Lethrinus mahsena from different sites in the tropical Indian Ocean: the banks of Seychelles, Mauritius and British Indian Ocean Territory (BIOT, Chagos Archipelago). The reliability of age–frequency distributions and individual ages estimated using otolith weight–age relationships was examined through comparison with those estimated through the standard method of ageing using otolith increments. Two other methods for estimating age–frequencies using age-slicing via an estimated growth curve were also examined; these used growth curves estimated by a length-based method (ELEFAN), or by fitting directly to length-at-age data (an ‘age-based’ method). Age-slicing using length-based growth parameters failed to produce reliable age–frequencies, due to inaccuracies in the growth parameter estimates. The use of age-based growth parameter estimates improved the results of age-slicing, however, age–frequencies remained significantly different from those obtained from ageing using otolith increments in two locations. The use of otolith weight–age relationships resulted in estimated age–frequency distributions that in all locations were not significantly different from those assessed through otolith increment counts. In contrast, L. mahsena otolith weight–age relationships could not be used to estimate individual ages accurately, due to the level of overlap in otolith weight between age classes. Where otolith increments are routinely used to age commercial fish species, the fact that otolith weight–age relationships could not be used to age individuals accurately may limit its application. However, where routine ageing of individuals through otolith increments is considered impractical, for instance because of its cost, the use of otolith weight–age relationships to derive catch age–frequencies represents a viable alternative approach. With this in mind, this study has also demonstrated that there is the potential to use otolith weight–age relationships for five other species caught around the Seychelles, following the validation of their otolith increments.

On a structural time series method for estimating stock biomass and recruitment from catch and effort data

Abstract Structural time series or state-space methods are well suited to stock assessment, where catch and effort are readily available observations, and population size is an unobserved component that one would like to describe. We discuss the use of such a model in estimating absolute abundance and recruitment from catch data and an index of abundance. We discuss the model in the context of the time series literature and the earlier fisheries models for such data. Model fitting is via maximum likelihood and the Kaiman Filter; the approach has advantages over traditional models in that it allows for both stochastic population dynamics and sampling error in index measurement. Further, it makes no assumptions about density-dependent growth or any functional relationship between recruitment and spawning stock size. We illustrate the method with an investigation of performance using simulated data and two sets of data pertaining to cod (Gadus morhua) in the North Sea. In the real examples the age-composition of the catch is available for part of the duration of the series. Virtual population analysis can therefore also be used; the results are found to be compatible with those from a time series approach.

Introduction: fisheries, past, present and future

That world fisheries are in a poor state is well known. Successive studies, many by authors of papers in this issue, have documented overexploitation, overcapitalization and threats to food security. The focus of this issue has been to reappraise fisheries and fisheries science, to look to the future and to indicate the challenges that exist both for science and for management if the future of fisheries is to be better than the past. Pauly et al. (2005) have examined broad historical trends in world fisheries and discussed, on a global scale, the deleterious impact that fisheries have had on marine ecosystems. Myers & Worm (2005) have focused on large predatory fishes, which have historically been the preferred target of fisheries and present particularly difficult conservation problems owing to their relatively sensitive demographic characteristics. Both studies, which build on earlier work, document the problems posed for science and management by the current situation and outline steps that need to be taken to relieve it. Garcia & Grainger (2005) have taken up the challenge to look very broadly to the future of marine capture fisheries and, in particular, to examine them in the context of recent global scenarios for the future world. The questions posed in their paper go beyond fisheries, and are relevant to the future of all industries based on natural resources. A central problem of fisheries lies in the mechanisms involved in fisheries’ management. Hilborn et al. (2005) review the relative success and failure of different institutional structures and argue that a key to success is that institutions need to be structured in such a way that incentives for individual fishing operators are consistent with conservation. Caddy & Seijo (2005) similarly examine the different roles of science and management, but focus on the real difficulties posed by the unpredictable nature of the response of marine ecosystems to exploitation and to the environment. In particular, they highlight the fragility of marine ecosystems and outline elements of a new management framework required to account for this. Cochrane & Doulman (2005) have looked at the problems posed for fishery management agencies by the plethora of legal or quasi-legal instruments developed by various international organizations over the past 20 years in an attempt to ensure sustainable use of natural resources. Although each instrument has made a positive contribution to sustainability, taken together they form a confusing and sometimes overwhelming set of obligations that places severe pressure on countries in both the developed and developing world that have subscribed to them. The authors examine these issues in the context of some of the future scenarios considered by Garcia & Grainger (2005). Central to some of the instruments examined by Cochrane and Doulman is the need for a more ecosystem-based approach to fisheries science and management. Mangel & Levin (2005) argue, with examples, that this implies the need to make community ecology, as opposed to population biology, the central scientific paradigm for fisheries science. A complementary theme is advanced by Pitcher (2005) in which he examines the feasibility of, and potential mechanisms for, returning marine ecosystems to something approaching their pristine or relatively unexploited structure. The need to consider ecosystems as a whole and the inadequacy, in many cases, of fishery management has led to a detailed examination of the concepts of marine-protected areas as management tools for conservation and sustainability. Roberts et al. (2005) examine, with examples, the effectiveness of marine reserves as a management tool. They argue that this effectiveness cannot be replicated by standard fishery management measures and therefore marine reserves must be incorporated into modern fishery management. Stefannson & Rosenberg (2005) examine the use of three management tools: closed areas, effort control and quota control. They conclude that the most efficient management, both for conservation and for economic benefit, is achieved by a combination of controls that includes relatively large closed areas. Quinn & Collie (2005), recognizing the demand for more ecosystem-based approaches to fishery management, argue nevertheless that the advances made, using population biology to provide stock assessment as the basic tool for fisheries science, need to be retained. They note the difficulty in applying more complex ecosystem models and examine ways in which the single-species approach has been adapted to meet the increasing needs for more conservative management. Beddington & Kirkwood (2005) address complementary problems, noting the great difficulties found in developing countries in obtaining and analysing data of sufficient detail to provide stock assessment and guide fisheries management. They have used life-history theory to derive much simplified assessment tools so that key elements of the stock assessment and management can be based on relatively sparse data. The majority of papers in this issue have concentrated on capture fisheries, but equally important to the future of fisheries and the issue of food security is the role of intervention in aquatic ecosystems, either by stock enhancement or aquaculture. Lorenzen (2005) examines the theory of stock enhancement in the context of practical management and develops tools for assessing the efficiency of particular enhancement programmes. Muir (2005) provides an overview of recent global trends in aquaculture and reviews its probable future and the scientific, environmental and economic problems that are likely to occur. The papers in this issue clearly indicate the types of institutional, managerial and scientific problems that face fisheries in the future. The future looks challenging.