River Otter, Lontra canadensis, Food Habits in the Missouri Ozarks

The reintroduction of River Otters (Lontra canadensis) between 1982 and 1992 resulted in widespread occurrence of the species throughout the Missouri Ozarks. This study examined otter diets from the vicinity of two Ozark streams in relation to seasonal and spatial trends. Otter scats (N = 4750) were collected and analyzed from the Osage Fork River and Big Piney River during the summer and winter seasons of 2001 and 2002. During the winter (January-March), fish occurred in 86% of the samples. During the summer (June–August), occurrence of fish dropped to approximately 15% for both rivers. Seven families of fish were identified in the diets, with Centrarchidae being most common regardless of river or season. Within the Centrarchidae, the genus Lepomis (mostly Longear Sunfish, Lepomis megalotis) was most common, with Micropterus (mostly Smallmouth Bass, Micropterus dolomieu) and Rock Bass (Ambloplites rupestris) also well represented. The mean age of Ambloplites consumed (mean = 3.3 years) was consistently older than that of either Micropterus (mean = 2.54 years) or Lepomis (mean = 2.78 years). Crayfish were recovered from a mean of 85.2% of scats in the winter and 99% in the summer. Smaller fish and crayfish were more common from the upper reaches of the streams while larger fish were prevalent in the lower reaches.

An evaluation of stable nitrogen isotopes and polychlorinated biphenyls as bioenergetic tracers in aquatic systems

This study investigated the relationship between stable nitrogen isotopes (δ15N) and polychlorinated biphenyl (PCB) concentrations in rock bass (Ambloplites rupestris) and bluegill sunfish (Lepomis macrochirus) with respect to age, size, and diet. δ15N signatures in both species exhibited enrichment with increasing size and approached steady state with respect to dietary δ15N values by the second year of growth. Young-of-the-year fish, however, exhibited lower nitrogen isotope enrichment over the diet, indicating that the commonly held trophic enrichment factor of 3.4‰ is more suitable for older, slow-growing individuals. PCB accumulation in both species progressed from being dominated by uptake from water in small (<100 g) fish to dietary uptake in larger individuals as a function of bioenergetic constraints such as food energy conversion and contaminant assimilation efficiencies. Significant increases in PCB accumulation were attributed to decreased specific growth rates such that ≤60% of body mass was gained on an annual basis. This effect was most noted in bluegills where higher PCB biomagnification factors were a consequence of increased foraging costs associated with an invertebrate diet. It is concluded that growth-related changes in species bioenergetics regulate both contaminant accumulation and δ15N dynamics.

Branching complexity and morphological characteristics of coarse woody structure as lacustrine fish habitat

The objective of this study was to quantify the physical characteristics of coarse woody structure (CWS) as fish habitat in a north temperate lake. Sixteen species of fish were observed in submerged CWS habitat. Branching complexity, distance above the bole, area below the bole, distance to other CWS, and water depth around CWS were significantly related to abundance of schooling cyprinids (Cyprinidae), rock bass (Ambloplites rupestris), smallmouth bass (Micropterus dolomieu), bluegill (Lepomis macrochirus), yellow perch (Perca flavescens), and walleye (Sander vitreus). Branching complexity was the most common characteristic of CWS related to richness, diversity, and total adult abundance of fish taxa, but was not correlated with the total lengths of fish found in submerged trees. Branching-complexity values ranged from 1 (simple) to 500 (moderately complex) in the littoral zone; for comparison, a living riparian conifer had a branching-complexity value of over 1000. Most CWS in the littoral zone was composed of simple trees without branching, but fish tended to inhabit CWS with branching-complexity values greater than 45. This study shows the importance of CWS with fine branching as littoral-zone fish habitat.

Indirect effects of lake acidification on Chaoborus population dynamics: the role of food limitation and predation

A twofold increase in Chaoborus punctipennis abundance following experimental acidification of one basin of Little Rock Lake, Wisconsin, provided an opportunity to examine the hypotheses that C. punctipennis dynamics were regulated by (i) food resources for instars I and II, (ii) fish predation on instars III and IV, and (iii) invertebrate predation on instars I and II. Neither food limitation nor fish predation appeared to be important constraints on C. punctipennis abundance. Although rotifer biomass increased significantly during the acidification, C. punctipennis did not respond in a consistent manner to variation in food availability. Bioenergetics estimates of C. punctipennis consumption by rock bass (Ambloplites rupestris), the most acid-sensitive fish species, were small compared with C. punctipennis production. In contrast, invertebrate predation appeared to be an important constraint on C. punctipennis abundance. Chaoborus punctipennis abundance increased dramatically when abundance of the predator Mesocyclops edax declined to near zero. Comparison of M. edax predation rates and first instar production suggested that M. edax could exert strong predation pressure on C. punctipennis Thus, invertebrate predation may create a juvenile bottleneck for populations of small-bodied Chaoborus species in moderately productive acidified lakes.

Mortality Risk of Spatial Positions in Animal Groups: the Danger of Being in the Front

We modified Hamilton's (1971) selfish herd model by introducing directional movement to the prey groups and the predators. The consequences of this modification with regards to differential predation risks are compared to Hamilton's original model (using stationary prey groups) and tested against empirical data. In model 1, we replicated Hamilton's original predator-prey system. In models 2 and 3, prey groups were mobile and predators were mobile (model 2) or stationary (model 3). Our results indicate that additional to the positive risk gradient from centre to periphery predicted by Hamilton's model for stationary groups, there might be another positive risk gradient from the rear to the front part in moving groups. Furthermore, models 2 and 3 suggest that moving groups should generally exhibit an elongated shape (longer than wide along the axis of locomotion) if risk minimisation is the only factor concerned. Also smaller inter-individual distances are predicted for front individuals than individuals elsewhere in the group. Empirical data based on the three-dimensional structure of fish shoals (using roach, Rutilus rutilus) were consistent with the above two predictions. A second experiment which involved lake chub, Semotilus atromaculatus, as prey and rock bass, Ambloplites rupestris, as predators, provided direct support for the hypothesis that individuals in front positions of groups incurred a significantly higher predation risk than fish in rear positions. Finally, we discuss the differential risks of different group positions in the context of potential foraging gains which provides the basis for a dynamic model of position preferences in group-living animals.