Figure 1. The four factors thought to matter in classical fish population dynamics. Note that, in this framework, the sole link of a given population to the other populations of fish, and to the ecosystem in general, is its natural mortality (M). Food consumption, required for growth (as per eqn [1]), and reproduction, required for recruitment, are usually not considered explicitly. Adapted from Russel (1931), and out of copyright).

Figure 2. Yield-per-recruit isopleth diagram for a southeast Asian red snapper, generated using eqn [3] (with the parameters W₁ 512. 2 kg; K 50.15 year ²¹; t₀ 520.67 year; M 50.33 year ²¹, b 53; and t_r 50 year) for different values of fishing mortality (F) and mean age at first capture (t_c), implying different body size and hence mesh sizes. Most fisheries tend to use meshes that are too small, and fishing mortalities that are too high, for the fish to be able to realize their growth potential (here over 300 g per recruit). Hence Y/R analysis often leads to the result, counterintuitive at first glance, that yield (Y) can be increased, whatever the number of recruits (R), by reducing fishing effort and increasing mesh sizes.

Figure 3. Schematic representation of the key economic factors affecting open-access fisheries. (a) Basic model, in which fishing costs are assumed to be proportional to fishing effort (f), and gross returns proportional to catches (parabola). (b) Under open access, f will increase past Maximum Economic Yield (MEY) at f₁ (where the economic rent, i.e. the difference between total costs and gross returns, is highest), and past Maximum Sustainable Yield (MSY, at f₂), until the equilibrium point (EP, at f₃), where costs and returns are equal, i.e. where the economic rent is completely dissipated. In this situation, subsidies, by reducing costs, increase the level of effort at which EP occurs, and thus decrease catches. (c) Price increases, by increasing gross returns, increase the level of effort at which the rent will be dissipated (i.e. from f₃ to f₄), and hence foster overfishing, just as subsidies do. (d) In small-scale fisheries, labour is a major cost factor; when its value tends toward zero (as occurs when there is a large excess of rural labour), resources may become severely depleted.

Figure 4. Schematic representation of ‘fishing down marine food webs’ (Pauly et al., 1998), wherein the fisheries, after depleting large, slow-growing fishes high in the food webs (with high trophic levels), increasingly come to rely on smaller, fast growing fishes (low trophic levels). This phenomenon occurs in most of the world's fisheries.