- The functioning of marine habitats needs to be understood in the context of the ecological relationships and associations between organisms and the physical and biogenic environment they inhabit. Thus, it becomes important to explore and define habitat features which contribute to these relationships and which are important in the life history of a species. Juvenile habitat needs and associations have rarely been studied, but Essential Fish Habitat (EFH) guidelines require description of habitat for all life stages of managed stocks. Given observational data in the natural environment, there is a strong need to determine if structural or biogenic habitat matters to fish. To explore direct relationships with habitat features, based on cause and effect, requires controlled laboratory studies. In this work, the extent and nature of habitat relationships for juvenile black rockfish, Sebastes melanops, were systematically investigated in four consecutive controlled experiments. I examined habitat choices of juvenile rockfish with respect to (1) a diurnal light cycle, (2) the nature of structure use with respect to the importance of crevices, (3) the effect of additional complexity in structure, and (4) the role of biotic components of structure independent of their physical attributes.
In the first experiment (1), juvenile rockfish behavior was video taped during day, crepuscular and night conditions. To observe fish behavior at night an infrared tank set-up was used, with lights projecting through the bottom of the tanks. The sand layer was required to be very thin, such that the light could penetrate through both tank wall and sand and get picked up by an overhead camera. This exposed many crevices and ledges at the base of the boulders which were extensively used by all groups of fish. In the second experiment (2), burying the boulders in deep sand reduced this crevice availability and took away those types of refuge opportunities. Complexity was then examined by attaching white plumed anemones, Metridium spp. to half of the boulders in the tanks (experiment 3). White plumed anemones occur as two species on the west coast of North America, Metridium farcimen and M. senile fimbriatum, which are visually indistinguishable and have overlapping distributions. For the purposes of our experiments, it was not necessary to identify them to species level, and all the experimental animals were collected by scuba in Yaquina Bay. Artificial replicates of Metridium spp. in comparison with live animals were used to test for biogenic contributions to habitat preference (experiment 4). To see if associations and behaviors observed in the laboratory also occur in the field, habitat use by unidentified reef-associated juvenile rockfish in their natural environment was examined using video data collected by the Oregon Department of Fish and Wildlife with a remotely operated vehicle.
The results of the experiments indicate that (1) the odds of juvenile black rockfish being present in boulders during daylight were 7.2 times greater than at nighttime. It is likely that refuge needs for juveniles of this species are greatest during daylight given the diversity of visual predators. (2) Crevice availability plays an important role in providing refuge, but juvenile black rockfish in my trials did not change boulder use based on crevice availability. (3) Juvenile black rockfish also clearly preferred habitat complexity (added anemones) and preferred boulders with attached Metridium spp. over plain boulders. (4) Preference was given not only to higher structural complexity, but also to habitat that offered a biotic component to the boulder/anemone arrangement. Juvenile black rockfish showed a significant association with boulder habitat containing attached live Metridium spp. over boulder habitat with the same structural construct using plastic replicas. The specific nature of this biogenic contribution to habitat choice is unknown and could be the result of an important symbiotic relationship. In their natural environment, unidentified juvenile rockfish also occurred in greater numbers in micro-habitats highly populated by Metridium spp. This work demonstrates that reef associated juvenile black rockfish engage in complex micro-habitat relationships that are not only based on structural complexity but also on biogenic influences. Biogenic habitats frequently involve invertebrate species that are fragile and long lived, and thus vulnerable to disturbance (e.g. from global warming, fishing activities and other extraction processes). Although survival and growth of juvenile black rockfish was not investigated in this work, the loss of biogenic habitat is likely to impact post settlement survival of reef-associated fishes, thus ultimately affecting population recruitment. Habitat limitations in the juvenile stage are particularly important to understand because density-dependent factors can dampen high stochasticity in recruitment to adult populations.