Seasonal and interannual variation in juvenile salmonids and associated fish assemblage in open waters of the lower Columbia River estuary

The transition between freshwater and marine environments is associated with high mortality for juvenile anadromous salmonids, yet little is known about this critical period in many large rivers. To address this deficiency, we investigated the estuarine ecology of juvenile salmonids and their associated fish assemblage in open-water habitats of the lower Columbia River estuary during spring of 2007–10. For coho (Oncorhynchus kisutch), sockeye (O. nerka), chum (O. keta), and yearling (age 1.0) Chinook (O. tshawytscha) salmon, and steelhead (O. mykiss), we observed a consistent seasonal pattern characterized by extremely low abundances in mid-April, maximum abundances in May, and near absence by late June. Subyearling (age 0.0) Chinook salmon were most abundant in late June. Although we observed interannual variation in the presence, abundance, and size of juvenile salmonids, no single year was exceptional across all species-and-age classes. We estimated that >90% of juvenile Chinook and coho salmon and steelhead were of hatchery origin, a rate higher than previously reported. In contrast to juvenile salmonids, the abundance and composition of the greater estuarine fish assemblage, of which juvenile salmon were minor members, were extremely variable and likely responding to dynamic physical conditions in the estuary. Comparisons with studies conducted 3 decades earlier suggest striking changes in the estuarine fish assemblage—changes that have unknown but potentially important consequences for juvenile salmon in the Columbia River estuary.

Motor task variation induces structural learning.

When we have learned a motor skill, such as cycling or ice-skating, we can rapidly generalize to novel tasks, such as motorcycling or rollerblading [1-8]. Such facilitation of learning could arise through two distinct mechanisms by which the motor system might adjust its control parameters. First, fast learning could simply be a consequence of the proximity of the original and final settings of the control parameters. Second, by structural learning [9-14], the motor system could constrain the parameter adjustments to conform to the control parameters' covariance structure. Thus, facilitation of learning would rely on the novel task parameters' lying on the structure of a lower-dimensional subspace that can be explored more efficiently. To test between these two hypotheses, we exposed subjects to randomly varying visuomotor tasks of fixed structure. Although such randomly varying tasks are thought to prevent learning, we show that when subsequently presented with novel tasks, subjects exhibit three key features of structural learning: facilitated learning of tasks with the same structure, strong reduction in interference normally observed when switching between tasks that require opposite control strategies, and preferential exploration along the learned structure. These results suggest that skill generalization relies on task variation and structural learning.

Habitat and diet overlap of 4 piscivorous fishes: variation on the inner continental shelf off New Jersey

Piscivorous fishes, many of which are economically valuable, play an important role in marine ecosystems and have the potential to affect fish and invertebrate populations at lower trophic levels. Therefore, a quantitative understanding of the foraging ecology of piscivores is needed for ecosystem-based fishery management plans to be successful. Abundance and stomach contents of seasonally co-occurring piscivores were examined to determine overlap in resource use for Summer Flounder (Paralichthys dentatus; 206–670 mm total length [TL]), Weakfish (Cynoscion regalis; 80–565 mm TL), Bluefish (Pomatomus saltatrix; 55–732 mm fork length [FL]), and Striped Bass (Morone saxatilis; 422–920 mm FL). We collected samples from monthly, fishery-independent trawl surveys conducted on the inner continental shelf (5–27 m) off New Jersey from June to October 2005. Fish abundances and overlaps in diet and habitat varied over this study period. A wide range of fish and invertebrate prey was consumed by each species. Diet composition (determined from 1997 stomachs with identifiable contents) varied with ontogeny (size) and indicated limited overlap between most of the species size classes examined. Although many prey categories were shared by the piscivores examined, different temporal and spatial patterns in habitat use seemed to alleviate potential competition for prey. Nevertheless, the degree of overlap in both fish distributions and diets increased severalfold in the fall as species left estuaries and migrated across and along the study area. Therefore, the transitional period of fall migration, when fish densities are higher than at other times of the year, may be critical for unraveling resource overlap for these seasonally migrant predators.

Diel movements of fishes linked to benthic seascape structure in a Caribbean coral reef ecosystem

Many common fishes associated with Caribbean coral reef ecosystems use resources from more than 1 patch type during routine daily foraging activities. Few studies have provided direct evidence of connectivity across seascapes, and the importance of benthic seascape structure on movement behavior is poorly known. To address this knowledge gap, we coupled hydro-acoustic technology to track fish with seafloor mapping and pattern analysis techniques from landscape ecology to quantify seascape structure. Bluestriped grunts Haemulon sciurus and schoolmaster snapper Lutjanus apodus were tracked over 24 h periods using boat-based acoustic telemetry. Movement pathways, and day and night activity spaces were mapped using geographical information system (GIS) tools, and seafloor structure within activity spaces was mapped from high-resolution aerial photography and quantified using spatial pattern metrics. For both fish species, night activity spaces were significantly larger than day activity spaces. Fish exhibited a daytime preference for seascapes with aggregate coral reef and colonized bedrock, then shifted to night activity spaces with lower complexity soft sediment including sand, seagrass, and scattered coral/rock. Movement path complexity was negatively correlated with seascape complexity. This demonstrates direct connectivity across multiple patch types and represents the first study to apply quantitative landscape ecology techniques to examine the movement ecology of marine fish. The spatially explicit approach facilitates understanding to the linkages between biological processes and the heterogeneity of the landscape. Such studies are essential for identifying ecologically relevant spatial scales, delineating essential fish habitat and designing marine protected areas.