Successful surf-riding on size spectra: the secret of survival in the sea J. G. POPE, J. G. SHEPHERD a n d J. WEBB M inistry ofAgriculture, Fisheries and Food, Directorate of Fisheries Research, Pakefield Road, Lowestoft, Suffolk NR33 OHT, U.K. SUMMARY All ecosystems require constituent species to survive against a backcloth of biotic and abiotic scenery. How this scenery shapes the life-history strategies of the players and how they in turn shape the scenery are important themes of the play of life. Species surviving in temperate and Arctic shelf seas do so against a scenery dominated by seasonal changes in the size-spectrum of other players. Successful survival in such an environment requires species to ride the big wave of annual productivity as it rolls through the extended size spectrum from phytoplankton to large fish. This wave flattens and broadens as it moves towards higher sizes. We speculate that in a seasonal shelf seas environment the wave shape is such that the Sheldon- Sutcliffe spectrum of equal biomass per log size interval is approximately true as an annual average although it may not be true at any particular moment in the year. Such spectra are structured by biomass being moved up the size spectrum mainly by predation processes, with growth of individuals being a second order process. However, the problem for an individual is to grow up through a size spectrum from its size at birth to its size at reproduction. Hence species need to find survival paths through the fluctuating scenery. This scenery is composed of the biomass of the prey, that of animals of a similar size, and larger predators. The paths followed dictate the life-history strategies of the species. This central problem for sea dwellers in temperate and Arctic shelf seas generates a broad similarity in the choice of life-history strategy for many key players over quite wide geographic areas of the globe. In these seas, strategies of high fecundity, high mortality and high growth rate are particularly common while strategies of low fecundity and parental care are rare for much of the size range. These seas also seem to favour longer trophic chains than terrestrial systems and either several generations per year or multiannual life cycles rather than annual cycles. 1. INTRODUCTION The ecosystem of the open sea is very, different from land-based ecosystems. Most marine primary pro- ducers are planktonic and uni-cellular. They are therefore small, and maintain a similar size through- out their short and turbulent lives. It is upon this substrate of small plant cells that the rest of life in the sea depends. M ulti-cellular animals must evolve a strategy for effective utilization of this source of food, and their predators must likewise use them. From these basic factors derives the enormous importance of size as a determinant of survival in the sea. As a result of this dependence on small plants, life in the sea has evolved a tall pyramid of organisms, spanning perhaps seven or more trophic levels between primary producers and top predators. The textbook terrestrial structure of plant, herbivore and carnivore, is rare as are marine analogues of small animals browsing on large plants or predators of similar sizes to their prey, at least in the planktonic ‘mainstream’. The trophic pyramid in the sea is not only tall, but Phil. Trans. R. Soc. Lond. B (1994) 343, 41-49 Printed in Great Britain 41 tangled. The great majority of the animals are carnivorous, and may in principle eat anything of the appropriate size, typically 2 to 3 orders of magnitude less in mass. Because of this cannibalism and cross- predation are common and the potential for interest- ing and volatile population dynamics is clear! In temperate and high latitudes, this system is in addition subjected to strong seasonal forcing. Although the classic picture of a spring bloom is not universally applicable, there are many regions, includ- ing those which support large and important commer- cial fisheries, where this is a real and important feature. To deal adequately with these complexities at the species level seems impossible with present metho- dologies. We are therefore led to consider size struc- tured descriptions of the problem. The present paper thus follows the path pioneered by Sheldon el al. (1972) as it seems to offer the best overview of what is going on in these complex ecosystems. To the Sheldon biomass size spectrum, we add the idea of seasonal variability and develop a simple mathematical cartoon of a seasonally per- turbed size spectrum. This describes the great annual wave of production in size-time space, and this sets the © 1994 The Royal Society and the authors Downloaded from https://royalsocietypublishing.org/ on 22 January 2022