Kevin M. Boswell

The Dynamics of Coordinated Group Hunting and Collective Information Transfer among Schooling Prey

Predator-prey interactions are vital to the stability of many ecosystems. Yet, few studies have considered how they are mediated due to substantial challenges in quantifying behavior over appropriate temporal and spatial scales. Here, we employ high-resolution sonar imaging to track the motion and interactions among predatory fish and their schooling prey in a natural environment. In particular, we address the relationship between predator attack behavior and the capacity for prey to respond both directly and through collective propagation of changes in velocity by group members. To do so, we investigated a large number of attacks and estimated per capita risk during attack and its relation to the size, shape, and internal structure of prey groups. Predators were found to frequently form coordinated hunting groups, with up to five individuals attacking in line formation. Attacks were associated with increased fragmentation and irregularities in the spatial structure of prey groups, features that inhibit collective information transfer among prey. Prey group fragmentation, likely facilitated by predator line formation, increased (estimated) per capita risk of prey, provided prey schools were maintained below a threshold size of approximately 2 m(2). Our results highlight the importance of collective behavior to the strategies employed by both predators and prey under conditions of considerable informational constraints.

A Semiautomated Approach to Estimating Fish Size, Abundance, and Behavior from Dual-Frequency Identification Sonar (DIDSON) Data

Abstract We present a semiautomated analytical approach incorporating both image and acoustic processing techniques to apply to dual-frequency identification sonar (DIDSON) data. Our objectives were (1) to develop a standardized analysis pathway in order to reduce the effort associated with counting, measuring, and tracking fish targets; and (2) to empirically obtain estimates of basic target information (e.g., size, abundance, speed, and direction of travel). Analyses were conducted on DIDSON data collected at three different locations (the Kenai River, Alaska; Mobile River, Alabama; and Port Fourchon, Louisiana) with different equipment and deployment configurations. We developed an efficient postprocessing approach that can be applied to a variety of data sets, independent of user and deployment method. For two of the three data sets analyzed, the estimates of fish abundance derived from DIDSON analyses were not significantly different from the manual counts of DIDSON files. The analyses produced estim...

Hydroacoustics as a Tool for Assessing Fish Biomass and Size Distribution Associated with Discrete Shallow Water Estuarine Habitats in Louisiana

We developed a relative index of fish biomass and size distribution in ultra-shallow waters (< 2 m) of Barataria Bay, Louisiana, based on the comparison of horizontal hydroacoustic data with gill net and push trawl catches in an effort to understand the role that habitat plays in both fish biomass and distribution. Exclosure net experiments indicated that the contribution of acoustic backscattering from sources other than fishes were negligible. Split-beam transducer, gill net, and push trawl sampling were conducted concurrently in Barataria Bay to provide information on fish composition and length distributions and for comparisons among gear types. Results suggest that acoustic fish biomass was generally higher in the low salinity stations and lower at the high salinity stations, at least in March 2004. We observed a greater mean length of fishes associated with oyster shell habitats when compared to adjacent sand-mud habitats. This paper demonstrates the utility of hydroacoustics as a tool to quantify relative acoustic fish biomass and size distribution associated with common estuarine habitats in ultra-shallow waters. This study also illustrates the potential of using acoustics for augmenting traditional sampling procedures.

Tail-Beat Patterns in Dual-Frequency Identification Sonar Echograms and their Potential Use for Species Identification and Bioenergetics Studies

Abstract We observed patterns in echograms of data collected with a dual-frequency identification sonar (DIDSON) that were related to the tail beats of fish. These patterns reflect the size, shape, and swimming motion of the fish and also depend on the fishs angle relative to the axis of the beam. When the tail is large enough to reflect sound of sufficient intensity and the body is angled such that the tail beat produces periodic changes in the range extent covered by the fish image, then the tail beat becomes clearly visible on echograms that plot the intensity maximum of all beams. The analysis of DIDSON echograms of a mix of upstream-migrating Chinook salmon Oncorhynchus tshawytscha and sockeye salmon O. nerka resulted in the separation of two groups: (1) fish of sockeye salmon size that swam with a tail-beat frequency (TBF) between 2.0 and 3.5 beats/s and (2) fish of Chinook salmon size with a TBF between 1.0 and 2.0 beats/s. There was no correlation between TBF and fish size within each group, whic...

School density affects the strength of collective avoidance responses in wild-caught Atlantic herring Clupea harengus: a simulated predator encounter experiment

An experimental study in a semi-controlled environment was conducted to examine whether school density in wild-caught Atlantic herring Clupea harengus affects the strength of their collective escape behaviours. Using acoustics, the anti-predator diving responses of C. harengus in two schools that differed in density were quantified by exposing them to a simulated threat. Due to logistical restrictions, the first fish was tested in a low-density school condition (four trials; packing density = 1.5 fish m(-3); c. 6000 fish) followed by fish in a high-density school condition (five trials; packing density = 16 fish m(-3); c. 60 000 fish). The C. harengus in a high-density school exhibited stronger collective diving avoidance responses to the simulated predators than fish in the lower-density school. The findings suggest that the density (and thus the internal organization) of a fish school affects the strength of collective anti-predatory responses, and the extent to which information about predation risk is transferred through the C. harengus school. Therefore, the results challenge the common notion that information transfer within animal groups may not depend on group size and density.

Evaluation of target strength–fish length equation choices for estimating estuarine fish biomass

In the Gulf of Mexico (GOM), fish biomass estimates are necessary for the evaluation of habitat use and function following the mandate for ecosystem-based fisheries management in the recently reauthorized Sustainable Fisheries Act of 2007. Acoustic surveys have emerged as a potential tool to estimate fish biomass in shallow-water estuaries, however, the transformation of acoustic data into an index of fish biomass is not straightforward. In this article, we examine the consequences of equation selection for target strength (TS) to fish length relationships on potential error generation in hydroacoustic fish biomass estimates. We applied structural equation models (SEMs) to evaluate how our choice of an acoustic TS–fish length equation affected our biomass estimates, and how error occurred and propagated during this process. To demonstrate the magnitude of the error when applied to field data, we used SEMs on normally distributed simulated data to better understand the sources of error involved with converting acoustic data to fish biomass. As such, we describe where, and to what magnitude, error propagates when estimating fish biomass. Estimates of fish lengths were affected by measurement errors of TS, and from inexact relationships between fish length and TS. Differences in parameter estimates resulted in significant differences in fish biomass estimates and led to the conclusion that in the absence of known TS–fish length relationships, Love’s (J Acoust Soc Am 46:746–752, 1969) lateral-aspect equation may be an acceptable substitute for an ecosystem-specific TS–fish length relationship. Based upon SEMs applied to simulated data, perhaps the most important, yet most variable, component is the mean volume backscattering strength, which significantly inflated biomass errors in approximately 10% of the cases.

Not So Fast: Swimming Behavior of Sailfish during Predator–Prey Interactions using High-Speed Video and Accelerometry

Billfishes are considered among the fastest swimmers in the oceans. Despite early estimates of extremely high speeds, more recent work showed that these predators (e.g., blue marlin) spend most of their time swimming slowly, rarely exceeding 2 m s(-1). Predator-prey interactions provide a context within which one may expect maximal speeds both by predators and prey. Beyond speed, however, an important component determining the outcome of predator-prey encounters is unsteady swimming (i.e., turning and accelerating). Although large predators are faster than their small prey, the latter show higher performance in unsteady swimming. To contrast the evading behaviors of their highly maneuverable prey, sailfish and other large aquatic predators possess morphological adaptations, such as elongated bills, which can be moved more rapidly than the whole body itself, facilitating capture of the prey. Therefore, it is an open question whether such supposedly very fast swimmers do use high-speed bursts when feeding on evasive prey, in addition to using their bill for slashing prey. Here, we measured the swimming behavior of sailfish by using high-frequency accelerometry and high-speed video observations during predator-prey interactions. These measurements allowed analyses of tail beat frequencies to estimate swimming speeds. Our results suggest that sailfish burst at speeds of about 7 m s(-1) and do not exceed swimming speeds of 10 m s(-1) during predator-prey interactions. These speeds are much lower than previous estimates. In addition, the oscillations of the bill during swimming with, and without, extension of the dorsal fin (i.e., the sail) were measured. We suggest that extension of the dorsal fin may allow sailfish to improve the control of the bill and minimize its yaw, hence preventing disturbance of the prey. Therefore, sailfish, like other large predators, may rely mainly on accuracy of movement and the use of the extensions of their bodies, rather than resorting to top speeds when hunting evasive prey.

Seasonal Estimates of Fish Biomass and Length Distributions Using Acoustics and Traditional Nets to Identify Estuarine Habitat Preferences in Barataria Bay, Louisiana

Abstract We conducted hydroacoustic, gill-net, and push trawl surveys to quantify changes in habitat-specific fish size and biomass in shallow (<2-m) estuarine waters of Barataria Bay, Louisiana, in order to evaluate essential fish habitat. Surveys were conducted monthly between June 2003 and May 2004 among regions located along a north–south salinity gradient. The fish length distributions derived from the gill-net and push trawl catches showed moderate concordance with the measured target strength distributions, indicating that our integrated approach more effectively characterized the fish community than using only a single gear type would have. Acoustic estimates showed that biomass was highest during fall (mean ± SE; 2.30 ± 0.27 g/m3) and next highest in spring (1.49 ± 0.20 g/m3), with relatively low biomass during summer (0.70 ± 0.14 g/m3) and winter (0.86 ± 0.14 g/m3); pelagic fish biomass from nets was low during winter (53.9 ± 14.9 grams per unit effort [gpue]) but relatively high in fall (846.1 ± 207.2 gpue), spring (774.3 ± 175.5 gpue), and summer (620.3 ± 140.7 gpue). Oyster habitat supported a greater biomass of pelagic fish (acoustic survey: 1.54 ± 0.15 g/m3; gill-net survey: 467.3 ± 81.0 gpue) than soft-bottom habitat (acoustic: 0.94 ± 0.11 g/m3; gill-net: 315.2 ± 54.8 gpue). Among regions, the greatest biomass of pelagic fish was observed at polyhaline stations (acoustic: 1.78 ± 0.19 g/m3; gill-net: 654.3 ± 136.5 gpue), followed by mesohaline (acoustic: 1.18 ± 0.15 g/m3; gill-net: 378.5 ± 79.1 gpue) and oligohaline stations (acoustic: 0.82 ± 0.12 g/m3; gill-net: 228.3 ± 50.2 gpue). Gill-net biomass was linearly related to the acoustic biomass estimates of small pelagic fish. The complementary, multigear approach proved to be useful in evaluating habitat use and may be particularly helpful in identifying and monitoring ecosystem reference points to evaluate change and in standardizing ecosystem-based assessment approaches.

Characterization of Atlantic Cod Spawning Habitat and Behavior in Icelandic Coastal Waters

The physical habitat used during spawning may potentially be an important factor affecting reproductive output of broadcast spawning marine fishes, particularly for species with complex, substrate-oriented mating systems and behaviors, such as Atlantic cod Gadus morhua. We characterized the habitat use and behavior of spawning Atlantic cod at two locations off the coast of southwestern Iceland during a 2-d research cruise (15–16 April 2009). We simultaneously operated two different active hydroacoustic gear types, a split beam echosounder and a dual frequency imaging sonar (DIDSON), as well as a remotely operated underwater vehicle (ROV). A total of five fish species were identified through ROV surveys: including cusk Brosme brosme, Atlantic cod, haddock Melanogrammus aeglefinus, lemon sole Microstomus kitt, and Atlantic redfish Sebastes spp. Of the three habitats identified in the acoustic surveys, the transitional habitat between boulder/lava field and sand habitats was characterized by greater fish density and acoustic target strength compared to that of sand or boulder/lava field habitats independently. Atlantic cod were observed behaving in a manner consistent with published descriptions of spawning. Individuals were observed ascending 1–5 m into the water column from the bottom at an average vertical swimming speed of 0.20–0.25 m s−1 and maintained an average spacing of 1.0–1.4 m between individuals. Our results suggest that cod do not choose spawning locations indiscriminately despite the fact that it is a broadcast spawning fish with planktonic eggs that are released well above the seafloor.

Habitat differences in the feeding ecology of red snapper (Lutjanus campechanus, Poey 1860): a comparison between artificial and natural reefs in the northern Gulf of Mexico

Red snapper (Lutjanus campechanus, Poey 1860) support a valuable commercial and recreational fishery in the northern Gulf of Mexico; however there is much debate as to the role of habitat, particularly reef structures, in the feeding ecology of this species. Furthermore, little information is available from fish collected on large natural reefs, such as those on the continental shelf edge, thought to be the historical center of abundance. Previous research indicates that little nutrition is derived directly from artificial reefs; rather the majority of prey comes from surrounding soft bottom habitat. The goal of this study was to determine if there are differences in the feeding ecology of red snapper between standing oil and gas platforms, toppled platforms designated as artificial reefs, and natural reefs on the continental shelf edge, using a combination of gut content and stable isotope analyses. Results indicate that fish dominated diets at all three sites, but that differences exist in the contribution of major prey items by percentage dry weight among habitats. Red snapper collected from standing platforms consumed primarily fish, squid, and shrimp, while greater amounts of crabs, shrimp, and other crustaceans were consumed at toppled platforms. On the natural reefs, diets varied the most, consisting of both fish and crustaceans. Stable isotope analyses suggest fish collected over the standing platforms are more enriched in δ15 N, indicating feeding at a higher trophic level than the other habitats. No differences were observed in mean values of δ13C or δ34S, indicating consistency in basal resources among habitats.