Cnida sequestration in aeolid nudibranchs: variability and retention time of sequestered cnidae in the opalescent sea slug, Hermissenda crassicornis (Gastropoda, Nudibranchia)

Aeolid sea slugs can isolate and store stinging cnidae obtained from their cnidarian prey, presumably for their own defence. There are 30 different varieties of cnidae, identified by their unique structure. The aeolids engulf the cnidae and store them in a functional state at the tips of their cerata. Although the process of cnida sequestration is reasonably well understood in aeolids, two critical questions remain: (1) are cnida types uniformly distributed among the cerata and (2) how long do sequestered cnidae persist? I collected opalescent sea slugs (Hermissenda crassicornis (Eschscholtz, 1831)) from Barkley Sound, British Columbia, Canada, and determined the cnida complements in four cerata per individual by microscope. The cnida complements differed between cerata from different body regions within the individual (values of Whittaker’s dissimilarity index from 2.5% to 36.3%). Furthermore, the cnidae varieties in low abundances are not consistently present within an individual. I also found that H. crassicornis fed a non-cnidarian diet lost cnidae over time, but the cnidarian-fed individuals did not: 3 of 10 H. crassicornis on the non-cnidarian diet lacked cnidae completely at 42 days. Future studies of cnida sequestration should be mindful that one ceras does not give an adequate representation of the distribution of sequestered cnidae.

Young-of-the-year recruitment does not predict the abundance of older age classes in black rockfish in Barkley Sound, British Columbia, Canada

Recruitment and connectivity are important criteria for designing effective marine protected areas, as coastal fish populations must be sustained by settling juveniles. However, patterns of recruitment are difficult to observe, and adults and juveniles may occupy distinct habitats. We examined patterns of adult black rockfish Sebastes melanops abundance with respect to habitat and spatiotemporal variability in recruitment of young-of-the-year (YOY) to determine how these variables influence population density in and around a rockfish conservation area (RCA). For most year classes, there was no relationship between the density of YOY and the density of 1 yr olds or the density of adults, and instead habitat variables such as topological complexity and the amount of rocky substrate predicted adult black rockfish abundance. For 1 year class of moderate abundance at the YOY stage but high abundance at the 1 yr old stage, a significant relationship between 1 yr olds and subsequent adults was observed. We surmise that overwinter survival of YOY fish may be an important determinant for year-class strength in black rockfish. Although a companion study found low recruitment of YOY inside the RCA, our data indicate that the density of many species of rockfish was higher inside the RCA. These results highlight how the density of adults can be determined by post-recruitment processes such as movement into suitable habitat and mortality, rather than by recruitment of YOY, and have implications for the design of marine reserve networks.

Modelling the impact of poaching on metapopulation viability for data-limited species

We developed a spatially explicit simulation model of poaching behaviour to quantify the relative influence of the intensity, frequency, and spatial distribution of poaching on metapopulation viability. We integrated our model of poaching with a stochastic, habitat-based, spatially explicit population model, applied it to examine the impact of poaching on northern abalone (Haliotis kamtschatkana) metapopulation dynamics in Barkley Sound, British Columbia, Canada, and quantified model sensitivity to input parameters. While demographic parameters remained important in predicting extinction probabilities for northern abalone, our simulations indicate that the odds of extinction are twice as high when populations are subjected to poaching. Viability was influenced by poaching variables that affect the total number of individuals removed. Of these, poaching mortality was the most influential in predicting metapopulation viability, with each 0.1 increase in mortality rate resulting in 22.6% increase in the odds of extinction. By contrast, the location and spatial correlation of events were less important predictors of viability. When data are limited, simulation models of poaching combined with sensitivity analyses can be useful in informing management strategies and future research directions.

The Early Marine Biology of the Hatchery/Wild Juvenile Salmonid (Oncorhynchus sp.) Community in Barkley Sound, Canada

We conducted 11 purse seine/beachseine surveys over the summers of 2000 and 2001 to learn about the migration timing, distribution, and diet of hatchery chinook (Oncorhynchus tshawytscha) and coho (O. kisutch), and wild chinook, coho, sockeye (O. nerka) and chum (O. keta) juvenile salmon, in Barkley Sound, West Coast Vancouver Island. Juvenile salmon partitioned Barkley Sound by time and space, and by diet except for hatchery and wild coho. The analysis of migration timing included historic data for 1987-89, and results showed that timing differed between species and was consistent over years. Sockeye and chum dominated the juvenile salmon community until mid-June and hatchery and wild chinook dominated subsequently. Fish tended to be dispersed contagiously. Results of correlation analyses of catch suggested that fish of different origins and species did not co-occur. The euphausiid Thysanoessa spinifera was an important prey item but different fish species selected different sizes of T. spinifera at different times. The diet overlap between hatchery and wild coho did not affect return. Migration timing for sockeye and wild coho seems to reflect a strategy to enter the ocean when the biomass of the size fraction of T. spinifera that each species selects is likely to be maximal. Descriptions of migration timing, fish interactions, and diet provide information which appears to be useful for learning about the biological basis of salmon return variability.