The effect of progressive hypoxia on school structure and dynamics in Atlantic herringClupea harengus

Abstract

The effect of progressive hypoxia on the structure and dynamics of herring (Clupea harengus) schools in laboratory conditions was investigated. The length, width and depth of schools of about 20 individuals were measured from video recordings to test the hypothesis that during hypoxia fish schools change their shape and volume. School shape (calculated as the ratios of length/depth, width/depth and length/width) did not change significantly during hypoxia. School length, width, depth, area and volume were all significantly increased at 20% oxygen saturation. Volume, area and width were more sensitive to hypoxia; volume and width were also increased at 25% and area at 30% oxygen saturation.The degree of position changing (shuffling) of individuals within the school was also analysed. Shuffling in normoxia was observed to occur largely through 'O-turn' manoeuvres, a 360°turn executed laterally to the school that allowed fishes in the front to move to the back. O-turn frequency during normoxia was 0.69 O-turns fish Ϫ1 min Ϫ1 but significantly decreased with hypoxia to 0.37 O-turns fish Ϫ1 min Ϫ1 at 30% oxygen saturation. Shuffling was also investigated by measuring the persistence time of individual herring in leading positions (i.e. the first half of the school). No significant changes occurred during hypoxia, indicating that the decrease in O-turn frequency does not affect shuffling rate during hypoxia, and that position shuffling in hypoxic conditions is mainly due to overtaking or falling back by individual fishes.School integrity and positional dynamics are the outcome of trade-offs among a number of biotic factors, such as food, predator defence, mating behaviour and various physical factors that may impose certain limits. Among these, our results indicate that oxygen level modulates schooling behaviour. Oxygen alters whole-school parameters at oxygen saturation values that can be encountered by herring in the field, indicating that oxygen availability is an important factor in the trade-offs that determine school volume. An increase in school volume in the wild may increase the oxygen available to each individual. However, shuffling rate is not affected by hypoxia, indicating that the internal dynamics of positioning is the result of the balance of other factors, for example related to the nutritional state of each individual fish as suggested by previous studies.

Vertical distribution of fish biomass in hypoxic waters on the Gulf of Mexico shelf

“…These results are in agreement with other studies of pelagic species that have demonstrated vertical avoidance of low-oxygen bottom water (Aku et al 1997, Keister et al 2000, Wanink et al 2001, Prince & Goodyear 2006. Additional effects of vertical avoidance behavior have also been demonstrated, including disruption of diel patterns of vertical migration (Schurmann et al 1998), altered schooling behavior (Domenici et al 2000(Domenici et al , 2002, and changes in patch structure in overlying normoxic waters (Taylor & Rand 2003). Further research is needed to determine the consequences of vertical avoidance of bottom water hypoxia for these other aspects of spatial distribution for fishes in the Gulf of Mexico.…”

Kinematics and energetic benefits of schooling in the labriform fish, striped surfperch Embiotoca lateralis

“…Although positional preferences have been observed in schools in relation to the physiological state of each individual (e.g. hunger level, Krause et al 1998), schooling is a dynamic behaviour, and most fish continuously change positions from trailing to leading (Krause 1993(Krause , 1994Domenici et al 2002). A relatively constant reshuffling rate was found in schools of herring Clupea harengus which decreased when fish were exposed to hypoxia (Domenici et al 2002).…”

“…hunger level, Krause et al 1998), schooling is a dynamic behaviour, and most fish continuously change positions from trailing to leading (Krause 1993(Krause , 1994Domenici et al 2002). A relatively constant reshuffling rate was found in schools of herring Clupea harengus which decreased when fish were exposed to hypoxia (Domenici et al 2002). For all individuals to take advantage of schooling, sufficient shuffling should occur such that each fish can spend a relatively large proportion of the time in a trailing position (i.e.…”

Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

“…As the dimensions of schools increase, O-turning, used in changing position within the school, is suppressed while shuffling rate remains constant. This behaviour results in increased overtaking of position within the school, perhaps leading to an increase in overall energy expenditure (Domenici, 2002). The increase in spatial extent of the school leads to a larger inter-individual distance causing lower communication and higher vulnerability to predators.…”