Designing reserve networks for fisheries management

Over the last decade or so, coastal marine reserves or no-take zones have been promoted as a means of managing fisheries (e.g. Holland & Brazee, 1996). This is another example where an understanding of landscape structure, and metapopulation dynamics, will be necessary to devise management strategies. Probably the most fundamental questions of reserve design are the fraction of coastline that should be set aside and the appropriate size (and number) of reserves needed in relation to the fishery management using no-take zones: metapopulation considerations

Scale and behavior measure

Within clover cells Stems visited per minute

Between clover cells Cells visited per minute Primary mode of movement

Plot-wide movement Mean step length (m) Displacement ratio

Introduced Harmonia axyridis

Native Coleomegilla maculata

0.10 t 0.04 Crawl

Table 15.5 Search behavior of introduced and native labybird beetles at different scales in experimental clover landscapes. Values are means ±1 SE. Each 16 X 16 m plot contains 256 cells (each 1 m2); clover cells are those cells in which clover was present. For individual ladybirds that made at least five cell transitions, plot-wide movements were quantified in terms of mean step length and displacement ratio. Displacement ratio is net displacement (straight-line distance) divided by overall path length. (After With et al., 2002.)

dispersal potential of the target species. Hastings and Botsford (2003) developed a simple deterministic model to answer these questions for a hypothetical species with characteristics that are most likely to benefit from no-take zones: one with sedentary adults and dispersing larvae. Their approach is based on the idea that altering the spacing and width of reserves changes the fraction of larvae that are retained within or exported from reserves (Figure 15.20). It is, of course, larval export that provides the basis for a sustainable yield from nonreserve areas.

The MSY problem can be stated as 'fix the level of larval retention within reserves, F, to preserve the species, and adjust the fraction of coastline in reserves, c, to maximize the number of larvae that settle outside the reserves (available as yield)'. Note that because F remains constant (something the modelers have chosen to assume), changing c means changing the width of reserves. Suppose that a value of F of 0.35 is deemed necessary to maintain the species. The solid line in Figure 15.20b shows how c and reserve width need to change to maintain an F of 0.35. The mathematical details of the model need not concern us but it turns out that although the largest yield is obtained when the reserves are as small as possible (the arrow in Figure 15.20b), so that larval export to fished areas is maximized, the yield is only slightly reduced as the reserve configuration moves away from this optimum. Thus, Hastings and Botsford (2003) argue that practical considerations, such as making reserves large enough to be enforced, can be allowed to play a major part in reserve design, as long as reserves are not so large (beyond the 'shoulder' of the curve in Figure 15.20b) as to significantly depress yield.

Although the model is a gross simplification, particularly in terms of the lack of any uncertainty or temporal or spatial heterogeneity, it usefully highlights some general considerations of importance and provides a starting point for more sophisticated and species-specific models to address the question of whether reserve networks will be useful for fisheries management.

In each of the sections of this chapter we have sought to build on relatively simple concepts by gradually adding more elements of realism. However, it should be remembered that even our most complex examples still lack realism in terms of the web of species interactions within which the target species are embedded. In fact, many management solutions have to be focused at a higher level of ecological organization - multispecies communities and whole ecosystems. We deal with the ecology of communities and ecosystems in Chapters 16-21 before considering ecological applications at this ecological level in Chapter 22.

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