Marine biologists entering the field of fish population dynamics and fish stock management for the first time, are often struck by the small fraction of our knowledge of marine biology currently being used in assessing marine fish stocks. There are historical reasons for this to be touched on later, but it is clear that fish stock assessment as a practical discipline, rests on the assumption that quantitative values can be given to the extent and rate of fish removals from an eco-system. Therefore, to a large extent, the tendency is to collect and analyse data on the quantity, size and age structure only for those fish stocks currently under active exploitation. The limitations of this approach are evident enough to field biologists, even for the relatively simple eco-systems of north temperate regions where much of fish population dynamics theory evolved, and some principal research concerns in this area are currently with environment-species, and interspecific interactions. These latter questions have of course been preoccupations of biologists for a long time, but although methods of quantifying food and feeding of fish have been well documented, e.g., Bagenal (1978), ways of representing and analysing the mass of data, especially on stomach contents, have been slower to develop.
The strength of the historical approach of arguing always from easily quantifiable variables, cannot be challenged, and considering that fisheries managers are by and large non-scientists, who are impatient with either qualitative or probabilistic statements, the methodologies that have tended to be adopted in fisheries are those that provide quantitative (deterministic) conclusions. An acquaintance with marine systems however, shows the value of understanding species interrelationships, and indicates that usually the best one should hope for is a statement giving rather wide probabilities around any quantitative result. Evidence is mounting that attempting to manage individual species independently is rarely feasible in the long term, especially for complex and diversified tropical systems, and we already know that the physical environment is variable in most ecosystems. With this realization, it is necessary to look for whole-system approaches which, despite having a high degree of uncertainty, also meet the criterion that they provide some qualitative or semi-quantitative guidelines for managers.
Unfortunately, as noted in the preamble to this manuscript, there are no simple, unambiguous guidelines or a single simple theoretical basis for management of multispecies fisheries (see e.g. Pauly and Murphy, 1982; and May 1984, for a sampling of the current range of opinions), and a high priority in practice must be given to empirical, descriptive approaches (e.g., Rigler, 1982). It is unfortunate also that the present state of development of marine ecological theory today does not lend itself to a simple narrative, since the discipline is by nature complex and rapidly developing, with a variety of modelling frameworks now in process of evaluation (see e.g., reports of the Dahlem Conferenzen, (Gulland and Garcia, 1984), the ICLARM/CSIRO Workshop (Jones, 1982; Platt, Mann and Ulanowicz, 1981). Nonetheless, we attempt here to put the more important ecological concepts in an overall framework, with the guiding principle given in Part II that the flow of organic matter and energy through oceanic food webs oceans is governed by basic thermodynamic principles. This does not imply however (Part II (1 and 2)) that we are dealing with thermodynamic stability - quite the contrary! Synthesized organics follow complex pathways (the food web), which are inevitably affected favourably or unfavourably, to a greater or lesser extent, when man harvests one or more components of the system.
Predicting the nature, extent and direction of such disruptions, is by and large, a matter of educated guesswork at present, although some clues are now emerging. As a general statement how-ever, we feel sure that any future improvement in the way marine resources are managed will only be possible if more ecological concepts are incorporated into fisheries management than at present, and that such considerations should be especially taken into account when contemplating significant changes in the way that a fishery is pursued.
We believe that familiarity with some of the main concepts of marine ecology is essential background for all those involved in management of marine resources, and even more urgently, for those concerned with stock assessment in developing countries, to whom this literature on applied ecology is often unavailable.
The main rationale then of this document is three-fold:
To introduce fisheries workers, especially in developing countries, to a number of important schools of thought in marine ecology and their more recent extensions.
To point the way to some possible alternative approaches which may extend existing ecological theory to the aid of fish stock assessment, especially in tropical and semi-tropical areas where multi-species approaches are essential. A more “ecological” or holistic approach to data collection seems necessary than appeared earlier to be the case.
To place particular emphasis on the concept of the food web, as an aid to scientific management: how to construct food webs and what tentative conclusions may be drawn from them.
Great emphasis has been placed throughout on a visual approach in the belief that even complex interrelationships can be more readily grasped in this way by a reader unfamiliar with mathematical notation. The “flow diagram” in particular, in its many practical applications in fisheries, proves a key to integrating many different disciplines in a way that is not readily possible from a linear textual account. The other easily visualized, but often-neglected, spatial relationships in fisheries are treated in Part II (3).
The text is therefore intended both as background reading for courses in fish stock assessment, and as background material to aid in decisions and priorities in fisheries research and management science. At the same time, the preliminary nature of many conclusions in marine ecology is stressed, in that a definitive framework for action that applies in all situations does not yet exist.