Lecture: John F. Caddy
Via Cervialto 3,
Aprilia, 04011,Latina,
Italy
jfcaddy@yahoo.co.uk
Engineering marine habitats for productivity: an experimental approach to investigating bottlenecks in life histories from a population dynamics perspective
Abstract: This study follows from theoretical considerations first described in Caddy and Stamatopoulos (1990), namely that for motile organisms dependent on cover, the shortage of cover suitable for a life history stage may lead to high mortality through predation. This could in turn cause bottlenecks and constrain adult biomass and fisheries production.
Population size may also be constrained by a lack of cover for juveniles, and by the food available in the foraging arena around structurally-complex cover, and not by an absolute shortage of trophic resources in the whole area. This has obvious consequences for the distribution of artificial reefs in an area. Indirect evidence that suitable cover is usually seriously limited in the marine environment comes from the rapid colonization of artificial reefs. Polovina (1991) concluded from this common observation that adding artificial structures in fished areas makes populations more vulnerable to exploitation.
If reefs are located inside a protected area however, this may contribute to increasing holding capacity of the reserve.
Two observations are suggested as a basis for theoretical studies on the relationship between population dynamics and cover: 1) for fish and motile invertebrates natural mortality rate at size typically descends rapidly with age, levelling to a plateau towards maturity; and, 2) a similar mortality configuration is predicted by supposing that fish are protected where cover with fractal characteristics is available, but risk predation mortality when displaced from cover or while foraging or migrating. The primary cause for rapid juvenile growth is suggested to be the need to reduce exposure to predation mortality while competing for limited cover with species of similar size, and this has important consequences for reef design.
Extensive observations on the numbers of reef fish at size in an unexploited reef area (Munro 1983) seems indirect evidence for cover limitation. Attempting to assess the ‘holding capacity’ of the natural habitat prior to installing artificial reefs suggests to first use a ‘fractal sampler’ (e.g. Beck 1995) to identify fish sizes for which natural bottlenecks occur near the proposed reef site. Reef design could then be adjusted to compensate for natural habitat deficiencies. The acadja (brush park) fish culture technique of West Africa (e.g., Hem and Avit 1994), demonstrates that fishery productivity is enhanced by adding structure to the marine habitat. Similar experimental approaches to optimizing overall productivity may be achieved by modifying the structural aspects of nearshore habitats.
Willian J. Lindberg
