The recent paper by Dahlgren et al. (2006), which builds on the earlier paper of Beck et al. (2001), proposes a classification system to identify important marine nursery habitats to aid in directing future research and to provide managers with a tool for the protection of important habitats. This work is important in recognising the need for a clearer nursery ground definition, and particularly in emphasising that the value of a nursery ground goes beyond the abundance or density of juveniles it supports (Beck et al. 2001, Sheaves & Molony 2001, Dahlgren et al. 2006). However, we believe that this approach is oversimplistic and does not account for many key aspects of nursery ground value. In particular, the approach focuses solely on one aspect of nursery ground function, the provision of a physical area of habitat occupied by juveniles, and one measure of importance, the proportion of individuals contributed by a nursery ground. Consequently, the nursery ground concept of Dahlgren et al. (2006) fails to (1) identify and account for the effects of scale, (2) recognise the importance of complexity and connectivity, (3) recognise the importance of ecosystems, resources and processes in supporting juveniles, and (4) recognise that the value of a nursery ground is a function of the reproductive output of individuals from the nursery and not just the numbers of individuals it provides. Reproductive output, not numbers of recruits. Dahlgren et al. (2006), and Beck et al. (2001) before them, measured the value of nursery grounds in terms of numbers contributed to adult populations, either the average number of individuals per unit area (Beck et al. 2001) or the proportion of individuals (Dahlgren et al. 2006). This approach relates to the value of a nursery from a purely exploitive, short-term, fisheries perspective; it does not recognise that—in an evolutionary, ecological and a sustainable fisheries sense—it is the contribution to the production of succeeding generations that determines real nursery-ground value. Through its effect on size and growth, the quality of a juvenile habitat can have a large influence on the lifetime reproductive output of an individual (Chigbu & Sibley 1994, Sedinger et al. 1995). Consequently, individuals from some habitats will be more fecund than those from others, so that any classification of the importance of habitats based only on the number of individuals recruiting to adult populations is likely to be invalid and misleading. Spatial scale. While Dahlgren et al. (2006) point out some of the shortcomings of Beck et al. (2001), which can lead to overlooking important habitats, they fail to clearly identify what they mean by ‘habitat’. Dahlgren et al. (2006) use 2 examples; one of fresh and brackish ‘habitats’ within 1 river system, the other of structural ‘habitats’ such as sand, seagrass and hard bottom. Both examples focus on aggregations of all structural units of particular types within a circumscribed area. However, it is unclear how far beyond this scale the concept is likely to be applicable. In the past, the concept of ‘nursery grounds’ has been discussed at a myriad of scales; a single type of habitat structure (e.g. mangrove or seagrass), a single geographic unit (e.g. a reef or an estuary), an aggregate of units (e.g. all the reefs in a reef complex, all seagrass beds in a bay), all units of one type (e.g. algal beds or estuaries in general). However, the importance, scope and meaning of ‘nursery ground’ all vary between scales. Additionally, the ability to unambiguously identify contributions and allocate them to particular habitat units differs between scales, as do the techniques available for determining nursery ground contribution. Without a clear definition of the range of scales at which the concept is applicable, the scales at which managers should apply the concepts of Dahlgren et al. (2006) are ambiguous.
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