External and internal grouping characteristics of juvenile walleye pollock in the Eastern Bering Sea

Size and shape patterns of fish groups are collective outcomes of interactions among members. Consequently, group-level patterns are often affected when any member responds to changes in their internal state, external state, and environment. To determine how groups of fish respond to components of their physical and ecological environment, and whether the response is influenced by a component of their external state (i.e., fish age), we used a multibeam system to collect three-dimensional grouping characteristics of 5 age categories of juvenile walleye pollock (age 1, age 2, age 3, mixed ages 1 and 2, and mixed ages 2 and 3) across the eastern Bering Sea shelf over two consecutive years (2009–2010). Grouping data were expressed as metrics that described group size (length, height), shape (roundness, spread), internal structure (density, internal heterogeneity), and position (depth, distance above bottom). Physical data (water temperature measurements) were collected with temperature-depth probes, and ecological data (densities of predators and prey − adult walleye pollock and euphausiids, respectively) were collected with an EK60 vertical echosounder. Juvenile pollock maintained a relatively constant shape, size-dependent density (number fish/mean body length3), and internal horizontal heterogeneity among age categories and in the presence of predators and prey. There were changes to group structure in the face of local physical forcing. Groups tended to move towards the seafloor when bottom waters became warmer, and groups became vertically shorter, denser, and had more variation in horizontal internal density as group depth increased. These results are explored in relation to the value and limitations of using multibeam data to describe how external and internal group structure map onto environmental influences

Information transfer, heterogeneity, and local environmental effects on emergent group patterns defining fish schools: perspectives from different scales of observation

Thesis (Ph.D.)--University of Washington, 2015It is widely understood why animals group, but much less is known about how animals group. Not all group structure provides functional benefit to the individuals therein; however, those group structures that do provide functional benefits help to explain how animals group. To determine whether group structure is functional, it is necessary to keep track of every individual and the corresponding group patterns over long periods of time. Because this typically requires two different scales of observation, I examined grouping behavior of fish from two different perspectives: the individual-up and the group-down. In the individual-up approach, I used giant danios, Devario aequipinnatus, in tank experiments, and manipulated the level of heterogeneity within various groups, expressed in the form of knowledge, to determine whether the level of heterogeneity within a fish group affects information transfer between individuals and the cohesiveness of the group. In the group-down approach, I examined in situ groups of juvenile walleye pollock, Gadus chalcogrammus, in the Gulf of Alaska and the Bering Sea to determine how groups of fish respond to their biological (i.e., predator and prey densities) and physical (i.e., water temperature, bottom depth) environment, and whether the response is influenced by the age/size of the fish. The results of the tank experiments indicate that heterogeneous groups of fish acted cohesively, and members within the group exhibited behaviorally integrated responses. That is, they adopted some behaviors from both knowledge sets -those behaviors that are the most costly to give up. However, there was a threshold when the group minority became hindered by conformity (i.e., when the group minority was ~ 20%). These heterogeneous groups exhibited behaviors only from the knowledge set of the majority. The in situ studies indicate that grouping behavior of fish in the wild is consistent with expectations based on predation and foraging theory, possibly influencing the distribution patterns of age classes and the grouping patterns of mixed-age groups. Additionally, these results indicate that there appears to be no structural cost to forming mixed-age groups; rather, mixed-age groups can provide advantages for smaller, more preyed upon fish. Together, the individual-up and group down-approach show that fish that form heterogeneous groups (with respect to knowledge or age of the fish) are cohesive and offer advantages to their members

External and internal grouping characteristics of juvenile walleye pollock in the Eastern Bering Sea

Size and shape patterns of fish groups are collective outcomes of interactions among members. Consequently, group-level patterns are often affected when any member responds to changes in their internal state, external state, and environment. To determine how groups of fish respond to components of their physical and ecological environment, and whether the response is influenced by a component of their external state (i.e., fish age), we used a multibeam system to collect three-dimensional grouping characteristics of 5 age categories of juvenile walleye pollock (age 1, age 2, age 3, mixed ages 1 and 2, and mixed ages 2 and 3) across the eastern Bering Sea shelf over two consecutive years (2009–2010). Grouping data were expressed as metrics that described group size (length, height), shape (roundness, spread), internal structure (density, internal heterogeneity), and position (depth, distance above bottom). Physical data (water temperature measurements) were collected with temperature-depth probes, and ecological data (densities of predators and prey − adult walleye pollock and euphausiids, respectively) were collected with an EK60 vertical echosounder. Juvenile pollock maintained a relatively constant shape, size-dependent density (number fish/mean body length3), and internal horizontal heterogeneity among age categories and in the presence of predators and prey. There were changes to group structure in the face of local physical forcing. Groups tended to move towards the seafloor when bottom waters became warmer, and groups became vertically shorter, denser, and had more variation in horizontal internal density as group depth increased. These results are explored in relation to the value and limitations of using multibeam data to describe how external and internal group structure map onto environmental influences