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