Clifford H. Ryer

Evaluating the role of fish behavior in surveys conducted with underwater vehicles

It is often assumed that visual survey data provide more accurate fish counts than conventional extractive gear. As a result, use of underwater vehicles to assess the abundance and distribution of fishes has increased rapidly over recent years. However, a review of observations reported for 48 demersal marine fish taxa showed that almost all respond in some way to underwater vehicles. Whether or not movements or changes in behavior affect survey bias is more difficult to assess. A simple conceptual model is presented to evaluate relationships between stimulus intensity, distances from the vehicle where reactions occur, and survey bias. Largest bias is caused by attraction or avoidance that occurs outside the field of cameras or observers. While light level and vehicle speed have been explored experimentally in a few cases, much remains to be learned about how bias varies among species, age groups, different vehicles, and operating conditions. Given poor understanding of survey bias, we recommend that surv...

Social mechanisms facilitating exploitation of spatially variable ephemeral food patches in a pelagic marine fish

Juvenile walleye pollock, Theragra chalcogramma, are more successful at exploiting spatially variable ephemeral food patches when foraging in groups. This may be attributable to local enhancement and to social facilitation of feeding motivation. Feeding responses of individuals were not independent when fish foraged for spatially variable ephemeral food patches, indicating social cues displayed by successful foragers informed other fish of where food was located (local enhancement). Fish initiated feeding sooner when grouped (social facilitation) than when alone. These results indicate that local enhancement aids individuals in locating food patches, while social facilitation aids in rapid patch exploitation.

Shifting the balance between foraging and predator avoidance: the importance of food distribution for a schooling pelagic forager

It is widely held that when predator avoidance conflicts with other activities, such as feeding, avoidance of predators often takes precedence. In this study, we examine how predation risk and food distribution interact to influence the schooling behavior and swimming speed of foraging juvenile walleye pollock, Theragra chalcogramma. Fish were acclimated to either spatially and temporally clumped, or spatially and temporally dispersed food for 3 weeks. Fish were then monitored while feeding in the absence and presence of predatory sablefish, Anoplopoma fimbria. Fish foraging for clumped food swam rapidly in a loose school when predators were absent, but swam more slowly and adopted more cohesive schooling in the presence of predators, trading-off foraging opportunity for decreased vulnerability to predators. Fish foraging for dispersed food swam about slowly and did not engage in cohesive schooling in either the absence or presence of predators. These fish accepted greater predation risk in order to continue foraging, suggesting that the cost of schooling, in terms of decreased foraging opportunity, was greater when food was dispersed than when it was clumped. This lower responsiveness to predators among fish receiving dispersed food demonstrates that predator avoidance does not always take precedence over other activities, but rather, that a balance is maintained between predator avoidance and feeding, which shifts as food distribution changes.

Influences of food distribution on fish foraging behaviour

Abstract Juvenile walleye pollock, Theragra chalcogramma , use group foraging, mediated by local enhancement, when searching for prey that are clumped in time and space. Local enhancement occurs when fish are attracted to the site where another fish has discovered food. Experiments were conducted to determine whether fish forage as individuals when food is widely dispersed in time and space. When acclimatized to dispersed food, juvenile walleye pollock foraged as individuals and did not respond to the food discovery of others. Some fish appeared to aggressively defend areas of their tank, a behaviour not seen in fish foraging for clumped food. In a second experiment, examining the role of prior experience in fish foraging, fish were first acclimatized to clumped food and then suddenly switched to dispersed food. Fish continued to use group foraging, mediated by local enhancement, with no change over 20 food discovery events. These results indicate that social aspects of fish foraging strategies are flexible, but are also influenced by prior experience with resource distribution.

Social behavior of juvenile chum salmon, Oncorhynchus keta, under risk of predation: the influence of food distribution

SynopsisSocial interactions can influence both foraging reward and vulnerability to predators. We examined social interactions in groups of juvenile chum salmon, Oncorhynchus keta, receiving food that was either spatially dispersed, with many food items appearing synchronously, or spatially clumped, with individual food items appearing asynchronously. These experiments were conducted both in the presence and absence of predators. when food was dispersed and predators were absent, juvenile chum formed schools and all individuals had access to food, despite frequent agonistic interactions. When predators were present, schooling and feeding continued, but agonistic interactions ceased. In contrast, when food was clumped, dominant fish utilized aggression to monopolize food regardless of whether predators were present or absent, resulting in decreased group cohesion. These results illustrate that food distribution and social interaction may play a role in determining how fish balance predation risk against foraging reward.

Influences of divergent behavioral strategies upon risk allocation in juvenile flatfishes

Animals balance feeding and anti-predator behaviors at various temporal scales. When risk is infrequent or brief, prey can postpone feeding in the short term and temporally allocate feeding behavior to less risky periods. If risk is frequent or lengthy, however, prey must eventually resume feeding to avoid fitness consequences. Species may exhibit different behavioral strategies, depending on the fitness tradeoffs that exist in their environment or across their life histories. North Pacific flatfishes that share juvenile rearing habitat exhibit a variety of responses to predation risk, but their response to risk frequency has not been examined. We observed the feeding and anti-predator behaviors of young-of-the-year English sole (Parophrys vetulus), northern rock sole (Lepidopsetta polyxystra), and Pacific halibut (Hippoglossus stenolepis)—three species that exhibit divergent anti-predator strategies—following exposure to three levels of predation risk: no risk, infrequent (two exposures/day), and frequent (five exposures/day). The English sole responded to the frequent risk treatment with higher feeding rates than during infrequent risk, following a pattern of behavioral response that is predicted by the risk allocation hypothesis; rock sole and halibut did not follow the predicted pattern, but this may be due to the limited range of treatments. Our observations of unique anti-predator strategies, along with differences in foraging and species-specific ecologies, suggest divergent trajectories of risk allocation for the three species.

Laboratory and Field Evidence for Structural Habitat Affinity of Young-of-the-Year Lingcod

Abstract Lingcod Ophiodon elongatus are an overexploited fisheries species in the Pacific Northwest. Although there has been extensive study of the habitat requirements of adults, particularly during spawning, relatively little is known about the habitat requirements of juveniles. Many juvenile fish use structural components of habitat to alleviate the risk of predation as well as for foraging. Laboratory experiments and a field tagging study were performed to investigate habitat preference in juvenile lingcod. Initial laboratory experiments demonstrated an affinity for structure (rock, shell, or seagrass) over bare sand habitats. An acoustic tagging study conducted in Yaquina Bay, Oregon, corroborated the basic habitat preferences seen in laboratory experiments. Juvenile lingcod showed a high degree of site fidelity, remaining in discreet areas of significant structural complexity, as evidenced by underwater video of benthic habitat. Results from both laboratory and field studies establish that juvenile ...