Ecological performance of OCS platforms as fish habitat off California.
Abstract
Significant Results:
Task 1: In this study, we used data from manned submersible studies conducted between 1995 and 2002 around seven platforms and over 80 deeper water reefs in southern and central California. We focused our analyses on bocaccio and cowcod, species that had been economically important to recreational and commercial fishermen and were declared over-fished by the National Marine Fisheries Service.
Overall, we observed 1,054 bocaccio of all sizes at platforms and 976 at reefs, and 125 cowcod at platforms and 134 at reefs. Among bocaccio estimated to be mature, we observed 313 bocaccio at platforms and 313 at natural reefs and 38 mature cowcod at platforms and 61 at reefs. Adult fishes were patchily distributed among both platforms and natural reefs. Fifty-seven percent of the natural reefs had mature bocaccio, as did four of seven platforms. Forty-two percent of natural reefs harbored mature cowcod, as did two of five platforms. Two platforms, Gail and Hidalgo, harbored relatively high densities of both species. Individuals of both species tended to be larger at the platforms, particularly those inhabiting Platform Gail.
Because both densities and mean lengths of both species were higher at Platform Gail than at any natural reef, their potential larval production at that platform was far higher than at any natural reef. We estimated that if Platform Gail (whose footprint is 0.5337 ha) were removed, it would be the equivalent of removing 12.57 ha of average-producing natural habitat in southern California for cowcod, or equivalent to removing 29.24 ha of average-producing natural habitat in southern California for bocaccio.
This study demonstrated that, in some instances, fishes at platforms provide significant amounts of larvae, numbers that may be of regional importance.
Task 2: The daily growth rates of YOY blue rockfish collected during the fall of 1999 at three platforms and three natural reefs were compared. There was no evidence that fish at platforms grew more slowly than those living on natural reefs. Comparison of growth rates between fishes living at Platform Holly and Naples Reef implied that YOY blue rockfish may, in some instances, grow faster at platforms.
Task 3: This research was conducted at two sites, Platform Holly and Naples Reef, that are located near each other in the Santa Barbara Channel using the painted greenling, Oxylebius pictus (Hexagrammidae), a small reef fish common to offshore platforms and kelp beds off California. The experiment showed that juvenile painted greenling residing at Naples Reef experienced a relative predation rate more than 2.5 times greater than the estimated rate experienced by juveniles residing at Platform Holly. This is likely due to the relatively low density of predators that live in platform midwaters compared to natural reefs. This implies that the large numbers of young fishes living around the platforms of southern California have lower mortality rates due to predation, than do these fishes inhabiting natural reefs.