The collective effects of fishing

16.2.5 The collective effects of fishing

Designing experiments to study fishing effects has always been difficult because it is never clear whether areas subject to different fishing pressure can be treated as replicates that respond more or less independently to fishing. Spatial designs have often been used to investigate the effects of fishing in the tropics, with reefs or islands treated as repli- cates. This approach was adopted by Samoilys (1988) and Jennings and Polunin (1996) to examine the effects of fishing on the structure of fish com- munities, and by McClanahan to look at the fish, algal and urchin interactions described previously.

The assumption that different reefs could be treated as semi-independent replicates was

thought to be reasonable because small (km 2 ) areas

protected from fishing, such as marine reserves, would harbour a diverse and abundant fish com- munity when adjacent reefs were heavily fished. In addition, fish communities on reefs that were sep- arated by a few km would respond differently to local fishing pressure. It was also assumed that the migrations or movements of adult reef-associated fishes between replicate areas were limited, and this was later confirmed with tagging studies (e.g. Holland et al. 1993; Zeller and Russ 1998). How- ever, the major weakness in understanding the level of connectivity between reefs was associated with the planktonic egg and larval stages (Jones et al., Chapter 16, Volume 1; Polunin, Chapter 14, this volume). These could return to their natal reef, making the effects of fishing localized, or be transported to reefs that were many tens to hun- dreds of km away, making the effects of fishing col- lective. Clearly, if the majority of larvae recuited to sites away from the natal reef, then spatial studies of fishing effects gave little insight into the longer- term impacts of fishing. Moreover, if we do not know the scale on which we should test for fishing effects, it is equally difficult to know the scale on which to impose management strategies.

Two recent studies made direct assessments of larval dispersal rates. Both concluded that rates of dispersal between reefs were low and that most larvae were retained close to the natal reef. Swearer et al. (1999) looked at the evidence for

larval retention of bluehead wrasse (Thalassoma bifasciatum ) at St Croix (US Virgin Islands) in the Caribbean. They used isotopic signatures in the otoliths to identify the source of larvae that were recruiting to the reefs, and showed that many had been retained close to the area where they were spawned. Jones et al. (1999) marked the otoliths of approximately 10 million developing Ambon damselfish (Pomacentrus amboinensis) eggs with tetracycline, a marker that fluoresces when the otolith is examined under ultraviolet light. All the eggs were marked on a reef at Lizard Island on the Great Barrier Reef, and they comprised 0.5– 2.5% of total Ambon damselfish egg production on this reef. Later in the year, juveniles recruiting to the reef were caught, and their otoliths were removed and viewed under ultraviolet light. The frequency of otoliths with marks suggested that 15–60% of juveniles returned to the natal reef and that long-distance dispersal was not usual.

Thus there is some evidence to suggest that fish populations are largely self-sustaining on reefs that are separated by >10km. This suggests that spatial designs can be useful for looking at long- term fishing effects and that even small and iso- lated marine reserves may be useful conservation tools for some reef fish species. (See also Polunin, Chapter 14, this volume, for further commentary on the degree to which fish move away from small areas.)