The importance of organic detritus in lotic ecosystems: Aspects of trophic interactions

Very little research has been carried out on detrital energetics and pathways in lotic ecosystems. Most investigations have concentrated on the degradation of allochthonous plant litter by fungi, with a glance at heterotrophic bacteria associated with decaying litter. In this short review, the author describes what is known of the detrition of plant litter in lotic waters, which results from the degradative activities of colonising saprophytic fungi and bacteria, and goes on to relate this process to those invertebrates that consume coarse and/or fine particulate detritus, or dissolved organic matter that aggregates into colloidal exopolymer particles. It is clear that many of the key processes involved in the relationships between the physical, chemical, biotic and biochemical elements present in running waters are very complex and poorly understood. Those few aspects for which there are reliable models with predictive power have resulted from data collections made over periods of 20 years or more. Comprehensive research of single catchments would provide a fine opportunity to collect data over a long period.

Physiological ecology of the ciliated protozoon Loxodes

Loxodes faces special problems in living close to the oxic-anoxic boundary. In tightly-stratified ponds like Priest Pot its optimum environment may be quite narrow and it can be displaced by the slightest turbulence. Loxodes cannot sense an O sub(2) gradient directly but its ability to perceive gravity allows it to make relatively long vertical migrations. It is also sensitive to light and oxygen and it uses these environmental cues to modulate the parameters of its random motility: in the dark, it aggregates at a low O sub(2) tension and in bright light it aggregates in anoxic water. The oxic-anoxic boundary is also a zone where O sub(2) may be a scarce and transient resource, but Loxodes) can switch to nitrate respiration and exploit the pool of nitrate that often exists close to the base of the oxycline.

An examination of spatial and temporal genetic variation in walleye pollock (Theragra chalcogramma) using allozyme, mitochondrial DNA, and microsatellite data

We used allozyme, microsatellite, and mitochondrial DNA (mtDNA) data to test for spatial and interannual genetic diversity in wall-eye pollock (Theragra chalcogramma) from six spawning aggregations representing three geographic regions: Gulf of Alaska, eastern Bering Sea, and eastern Kamchatka. Interpopulation genetic diversity was evident primarily from the mtDNA and two allozyme loci (SOD-2*, MPI*). Permutation tests ˆindicated that FST values for most allozyme and microsatellite loci were not significantly greater than zero. The microsatellite results suggested that high locus polymorphism may not be a reliable indicator of power for detecting population differentiation in walleye pollock. The fact that mtDNA revealed population structure and most nuclear loci did not suggests that the effective size of most walleye pollock populations is large (genetic drift is weak) and migration is a relatively strong homogenizing force. The allozymes and mtDNA provided mostly concordant estimates of patterns of spatial genetic variation. These data showed significant genetic variation between North American and Asian populations. In addition, two spawning aggregations in the Gulf of Alaska, in Prince William Sound, and off Middleton Island, appeared genetically distinct from walleye pollock spawning in the Shelikof Strait and may merit management as a distinct stock. Finally, we found evidence of interannual genetic variation in two of three North American spawning aggregations, similar in magnitude to the spatial variation among North American walleye pol-lock. We suggest that interannual genetic variation in walleye pollock may be indicative of one or more of the following factors: highly variable reproductive success, adult philopatry, source-sink metapopulation structure, and intraannual variation (days) in spawning timing among genetically distinct but spatially identical spawning aggregates.

Distribution and co-occurrence of rockfishes (family: Sebastidae) over trawlable shelf and slope habitats of California and southern Oregon

The rockfishes of the sebastid genus Sebastes are a very important fishery resource off the coasts of California and southern Oregon. How-ever, many of the 54 managed stocks of west coast rockfish have recently reached historically low population levels, leading fishery managers to re-examine current management practices. Management of rockfish stocks as multispecies aggregates, as opposed to independent stocks within the ground-fish fishery, can be more desirable when nontargeted bycatch, discard, and management complexity are considered. Rockfish assemblage structure and species co-occurrences were determined by using data from the Alaska Fisheries Science Center triennial continental shelf bottom trawl survey. The weight of rockfish species in trawl catches was expressed as a catch-per-unit-of-effort (CPUE) statistic, from which species spatial distributions, overlaps, diversity, and richness were analyzed. Multidimensional scaling of transformed CPUE data was employed in indirect gradient and multivariate partitioning analyses to quantify assemblage relationships. Results indicated that rockfish distributions closely match the bathymetry of coastal waters. Indirect gradient analysis suggested that depth and latitude are the principal factors in structuring the spatial distributions of rockfish on trawlable habitat. In addition, four assemblages were identified through the joint evaluation of species’ distributions and multivariate partitioning analyses: 1) deep-water slope; 2) northern shelf; 3) southern shelf; and 4) nearshore. The slope, shelf, and near-shore groups are found in depth ranges of 200–500 m, 100–250 m, and 50–150 m, respectively. The division of northern and southern shelf assemblages occurs over a broad area between Cape Mendocino and Monterey Canyon. The results of this analysis are likely to have direct application in the management of rockfish stocks off the coasts of southern Oregon and California.