Estimated Prey Consumption Research Articles

Detecting narwhal foraging behaviour from accelerometer and depth data using mixed-effects logistic regression

BackgroundDue to their Arctic habitat and elusive nature, little is known about the narwhal (Monodon monoceros) and its foraging behaviour. Understanding its ability to catch prey is essential for understanding its ecological role, but also to assess its ability to withstand climate changes and anthropogenic activities. Narwhals produce echolocation clicks and buzzing sounds as part of their foraging behaviour and these can be used as indicators of prey capture attempts. However, acoustic data are expensive to store on the tagging devices and require complicated post-processing. The main goal of this paper is to predict prey capture attempts directly from acceleration and depth data. The aim is to apply broadly used statistical models with interpretable parameters. The ultimate goal is to be able to estimate prey consumption without the more demanding acoustic data.ResultsWe predict narwhal buzzing activity using mixed-effects logistic regression models with 83 features extracted from acceleration and depth data as explanatory variables. The features encompass both instantaneous values as well as delayed values to capture behavioural patterns lasting several seconds. The data correlations were not strong enough to predict the exact timing of the buzzes, but were reliably able to detect buzzes within a few seconds. Most of the of the buzz predictions were within 2 s of an observed buzz (68%), increasing to 94% within 30 s. Conversely, 46% of the observed buzzes were within 2 s of a predicted buzz, increasing to 82% within 30 s. Additionally, the model performed well, although with a tendency towards underestimation of the number of buzzes per dive. In total, we predicted 17, 557 buzzes versus 25, 543 observed across data from 10 narwhals. Classifying foraging and non-foraging dives yielded a precision of 86% and a recall of 91%.ConclusionWe conclude that narwhal foraging estimation through acceleration and depth data is a valid alternative or supplement to buzz recordings, even when using somewhat simple statistical methods, such as logistic regression. The methods in this paper can be extended to foraging detection in similar marine species and can aid instrument development.

Temporal variation in translocated Isle Royale wolf diet.

Wolves (Canis lupus) can exert top-down pressure and shape ecological communities through the predation of ungulates and beavers (Castor spp.). Therefore, understanding wolf foraging is critical to estimating their ecosystem-level effects. Specifically, if wolves are consumers that optimize tradeoffs between the cost and benefits of prey acquisition, changes in these factors may lead to prey-switching or negative-density dependent selection with potential consequences for community stability. For wolves, factors affecting cost and benefits include prey vulnerability, risk, reward, and availability, which can vary temporally. We described the wolf diet by the frequency of occurrence and percent biomass and characterized the diet using prey remains found in wolf scats on Isle Royale National Park, Michigan, USA, during May-October 2019 and 2020. We used logistic regression to estimate prey consumption over time. We predicted prey with temporal variation in cost (availability and/or vulnerability) such as adult moose (Alces alces), calf moose, and beaver (Castor canadensis) to vary in wolf diets. We analyzed 206 scats and identified 62% of remains as beaver, 26% as moose, and 12% as other species (birds, smaller mammals, and wolves). Adult moose were more likely to occur in wolf scats in May when moose are in poor condition following winter. The occurrence of moose calves peaked during June-mid-July following birth but before calf vulnerability declined as they matured. By contrast, beaver occurrence in wolf scat did not change over time, reflecting the importance of low-handling cost prey items for recently introduced lone or paired wolves. Our results demonstrate that the wolf diet is responsive to temporal changes in prey costs. Temporal fluctuation in diet may influence wolves' ecological role if prey respond to increased predation risk by altering foraging or breeding behavior.

Differentiated payments for ecosystem services based on estimated prey consumption by lions within communal conservancies in northwest Namibia

When effectively applied, differentiated payments for ecosystem services (DPES) can help offset certain costs incurred by communities living alongside destructive wildlife. In areas with human-lion conflict (HLC), strategies for addressing the costs of living with large carnivores have primarily focused on compensation payments for lost livestock, but a more complete approach would include the value of prey species consumed by lions that might otherwise have market value for local communities. We introduce an approach for translating the value of prey species consumed by lions from opportunity costs into DPES as one approach for assessing the costs of coexistence with lions. Because lions are unequally distributed across the landscape, efficient DPES require spatially explicit lion movement data. Using data from GPS-collared lions, we link the movements of five lions within six communal conservancies in northwest Namibia to predation rates to estimate the differentiated opportunity costs to each conservancy in the form of wild prey species consumed by lions. Using two population estimates, we show how movement and predation data could be scaled up and suggest applications for addressing other human-wildlife scenarios.

Modeling Brook Trout Carrying Capacity in Owhi Lake, Washington, Using Bioenergetics

AbstractThe management of fish populations often requires an understanding of how density‐dependent effects influence population dynamics. In systems where natural populations are supplemented with stocking, the question of “how much food is available” becomes increasingly important. One typical approach for assessing density‐dependent interactions is to identify disparities between fish consumption rates and food availability. The objective of our study was to determine whether seasonal lake prey production could support Brook Trout Salvelinus fontinalis consumption demand in Owhi Lake, Washington, at observed abundances. Brook Trout were collected seasonally from 2015 to 2017 to obtain information on length, weight, age, diet, growth, and mortality. Population abundance was estimated in summer by using hydroacoustic surveys. Littoral invertebrates and pelagic zooplankton were collected concurrently with fish to enumerate biomass and production. Bioenergetics modeling was used to estimate prey consumption by Brook Trout. In conjunction with supply–demand comparisons, we used growth efficiencies and maximum consumption rates to further identify potential seasonal and annual food limitations. Our results suggest that prey production could support Brook Trout consumption demand for all years, but littoral invertebrate consumption was close to or exceeded prey production in summer and fall 2017. Growth efficiency was lowest and maximum consumption rates were highest in summer 2017 relative to all other seasons and years. In addition to observed diet switching from littoral invertebrates to zooplankton in summer 2016 and 2017, we concluded that lower growth efficiencies, lower annual survival rates, and increased consumption rates were influenced by littoral invertebrate production. The Owhi Lake Brook Trout stocking program may require adaptive management (i.e., annual evaluations) to balance natural recruitment.

A diverse suite of pharmaceuticals contaminates stream and riparian food webs

A multitude of biologically active pharmaceuticals contaminate surface waters globally, yet their presence in aquatic food webs remain largely unknown. Here, we show that over 60 pharmaceutical compounds can be detected in aquatic invertebrates and riparian spiders in six streams near Melbourne, Australia. Similar concentrations in aquatic invertebrate larvae and riparian predators suggest direct trophic transfer via emerging adult insects to riparian predators that consume them. As representative vertebrate predators feeding on aquatic invertebrates, platypus and brown trout could consume some drug classes such as antidepressants at as much as one-half of a recommended therapeutic dose for humans based on their estimated prey consumption rates, yet the consequences for fish and wildlife of this chronic exposure are unknown. Overall, this work highlights the potential exposure of aquatic and riparian biota to a diverse array of pharmaceuticals, resulting in exposures to some drugs that are comparable to human dosages.

Evaluating the functional importance of secretive species: A case study of aquatic snake predators in isolated wetlands

AbstractAlthough the need to prioritize limited conservation resources has prompted increased interest in understanding the functional importance of species within ecosystems, species that are infrequently observed are often written off as being unimportant. In this study, we use aquatic snakes as a case study for examining the importance of secretive predators. Most snakes are extremely cryptic and secretive, traits that not only lead to the perception that they are rare, and of minor importance, but also impede attempts to quantify densities. We used high sampling effort and robust‐design capture–recapture analyses to estimate density of aquatic snakes inhabiting an isolated 5.4‐ha wetland in South Carolina, USA. We assessed snake diets and coupled field measurements of growth rates with laboratory‐derived data on mass conversion efficiency to estimate prey consumption by snakes over a 1‐year period. We found a peak density 171 snakes ha−1 of wetland habitat, corresponding to a standing biomass of 7.77 kg ha−1. We calculated that snakes within the wetland consumed a total of over 200 kg (>55 000 individuals) of amphibian prey annually, translating into >150 000 kJ ha−1 of energy flow from secondary to tertiary consumers within the wetland food web. Further, because many amphibians are primarily terrestrial as adults and are consumed by aquatic snakes only when they return to wetlands to breed, snakes can be responsible for substantial transfer of energy and biomass between terrestrial and aquatic habitats. Our study is one of the first comprehensive evaluations of the importance of snakes as predators and underscores the need to consider snakes in initiatives aimed at preserving overall ecosystem integrity.

Digestion and predation rates of zooplankton by the pleustonic hydrozoanVelella velellaand widespread blooms in 2013 and 2014

Surface-dwelling colonies of Velella velella occur throughout tropical to cold-temperate oceans of the world and sometimes are stranded in masses along hundreds of kilometers of beaches. Large-scale blooms in the Western Mediterranean Sea in 2013 and 2014 allowed the study of diet, prey digestion times and predation rates. Gastrozooid content analyses showed that 59% of the 769 identified prey were euphausiid larvae (calytopsis and furcilia) captured at night. Copepods (41%), fish eggs (2.2%) and larvae (0.5%) were captured both at day and night. Digestion times at ambient temperature ( 178C) of calytopsis, furcilia and copepods were estimated to be .6.5, 4.4 and 3.9 h, respectively. Estimated prey consumption was substantially lower in 2014 than in 2013 (41 vs. 75 prey day colony). Velella velella and other gelatinous species bloomed in the Mediterranean Sea and the northeastern Atlantic and Pacific oceans in 2013 and 2014. Because of the wide distribution of V. velella colonies, their mass occurrences, potential importance as predators and competitors of fish, additional production from symbiotic zooxanthellae and stranding on beaches, they could be important in open-ocean carbon cycling and in transport of pelagic production to landmasses.

A bioenergetics model for estimating prey consumption by an Adélie penguin population in East Antarctica

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 526:183-197 (2015) - DOI: https://doi.org/10.3354/meps11182 A bioenergetics model for estimating prey consumption by an Adélie penguin population in East Antarctica Darren Southwell1,3,*, Louise Emmerson1, Jaume Forcada2, Colin Southwell1 1Australian Antarctic Division, Department of the Environment, Channel Highway, Kingston, TAS 7050, Australia 2British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK 3Present address: School of Botany, University of Melbourne, VIC 3010, Australia *Corresponding author: darren_southwell@hotmail.com ABSTRACT: Quantifying prey consumption by top predators is a crucial component of ecosystem-based management in the Southern Ocean. In this study, we developed a bioenergetics model to estimate prey consumption by a top predator, the Adélie penguin Pygoscelis adeliae. Our model predicts prey consumption throughout the breeding season and incorporates uncertainty in model parameters using Monte Carlo simulation. The model was parameterized with data obtained at Béchervaise Island, the site of a long-term monitoring program in East Antarctica. We parameterized the model (1) using 13 yr of penguin population data, (2) for a year in which penguins successfully reared their chicks (2001-2002) and (3) for a year with low breeding success (1998-1999). Daily per capita energy consumption during the breeding season averaged 4269 kJ d-1 (95% CI: 4187-4352 kJ d–1) and 4684 kJ d-1 (95% CI: 4596-4771 kJ d–1) for males and females, respectively. Over the entire breeding season a male breeder consumes 470 MJ (95% CI: 461-479 MJ) compared to 515 MJ (95% CI: 506-525 MJ) for a female. On average, the Béchervaise Island population of 1836 breeding pairs consumes 16447 MJ d-1 which amounts to 1809224 MJ during the breeding season. On the basis of variable breeding success and the proportion of krill and fish in their diet, we estimate that this population consumes 78 to 406 t of krill and 4 to 46 t of fish each breeding season. Our results demonstrate clear periods of peak consumption associated with the penguins’ breeding cycle. KEY WORDS: Southern Ocean · Predator–prey · Uncertainty · Krill · Harvesting · Fisheries Full text in pdf format Supplementary material PreviousNextCite this article as: Southwell D, Emmerson L, Forcada J, Southwell C (2015) A bioenergetics model for estimating prey consumption by an Adélie penguin population in East Antarctica. Mar Ecol Prog Ser 526:183-197. https://doi.org/10.3354/meps11182 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 526. Online publication date: April 22, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Prey-field use by a Southern Ocean top predator: enhanced understanding using integrated datasets

An important challenge for understanding and managing marine ecosystems is determining the relationship between the distributions of prey species and the foraging of top predators. We examined the diet and foraging dynamics of breeding macaroni penguins Eudyptes chrysolophus from sub-Antarctic Heard Island and related these to prey distributions derived from active-acoustics and net-derived data in the foraging zone of the penguins. Consistent with previous findings, we found that penguin diets changed between the guard and creche stages of the breeding cycle and that this change in diet corresponded with a switch from short foraging trips in the guard stage to significantly longer, offshore foraging trips in the creche stage. We related these differences in diet and foraging to characteristics of the prey field—specifically, a relatively uniform distribution of krill over the shelf and in deeper, offshore waters, compared with an increasing abundance of fish further from shore. We developed a simple dynamic energy budget for macaroni penguins to explore whether targeting fish during the creche stage was an energetically favourable strategy. Finally, we extrapolated our energy budget to estimate prey consumption at the colony scale as previous work has suggested that depletion near breeding colonies could contribute to prey switching. We found that prey switching during the creche stage was energetically favourable and was most likely related to a reduction in foraging constraints, rather than prey depletion. This study shows the value of integrating data sets to address questions surrounding variation in diet and the use of alternative prey by marine predators.

Potential impacts of climate change on growth and prey consumption of stream‐dwelling smallmouth bass in the central United States

AbstractSmallmouth bass (Micropterus dolomieu, SMB) is a broadly distributed, economically important species in the USA and Canada. Although previous research has suggested that projected climate warming may allow SMB to thrive beyond their current northern distribution, little research has been devoted to the population‐level effects of climate change on warm‐water fishes, including SMB. We modelled the impacts of projected climate change on growth of stream‐dwelling SMB along a north–south gradient in the central USA. Using downscaled regional projections from three global climate models, we generated scenarios for thermal habitat change for four populations (in Oklahoma, Missouri, Iowa and Minnesota) and used bioenergetics simulations to estimate prey consumption and growth under future projections. Bioenergetics simulations showed that prey consumption is expected to increase in all populations with moderate stream warming (2–3 °C). Growth potential is predicted to increase by 3–17% if not limited by food availability with stream warming by 2060 and was most pronounced for southern populations. For each 1 °C increase in stream temperature, SMB consumption would be expected to increase by about 27% and growth would increase by about 6%. Due to implications for species interactions, population performance and regulation of local fisheries, a better understanding of how SMB populations will respond to climate change is recommended for effective management and conservation.

Diet and prey consumption of breeding Common Kestrel ( Falco tinnunculus) in Northeast China

The diet of Common Kestrels Falco tinnunculus was studied during the breeding seasons from 2004 to 2008 in Northeast China. Diet was determined by direct observation of the prey brought to nests, and analysis of prey remains collected from nests. Fifteen vertebrate species from three classes, and two groups of insects were identified as prey items. Rodents were the main prey items, comprising 93.9% of the total prey items (TPI) and 97.0% of total prey biomass (TPB). Birds, frogs and insects were also eaten. The kestrels preferred to prey on small rodents (mean weight 20–40 g) and displayed density-dependent prey selection. Daily prey consumption of an adult and a nestling was 2.6 individual rodents or 87.6 g, and 1.7 individual vertebrates or 48.2 g, respectively. The estimated prey consumption of a breeding pair (adults and nestlings) during the breeding season was 520.1 individual vertebrates or 19.7 kg.

Evaluating the prudence of parents: daily energy expenditure throughout the annual cycle of a free‐ranging bird, the macaroni penguin Eudyptes chrysolophus

We measured daily energy expenditure (DEE) continuously for a whole year in a free ranging bird, the macaroni penguin Eudyptes chrysolophus. We combined these measurements with concurrently recorded foraging behaviour, and literature information on body mass and dietary factors to estimate prey consumption rates and foraging success. DEE was at a maximum during late chick‐rearing but was equally high during all other active phases of the breeding season. DEE was approximately 4×resting metabolic rate, which accords with established theory and suggests a common ‘energetic ceiling’ throughout the summer period. However, whether this represents a maximum in physiological capacity, or a rate which optimises fitness is still unclear. Rates of prey consumption and foraging success followed different patterns from daily energy expenditure. Daily prey consumption was high as the penguins prepared for long fasts associated with moulting and incubation but relatively low during chick‐rearing, when foraging areas were restricted and foraging success lower. It appears that the energy intake of macaroni penguins is subject to extrinisic or environmental constraints rather than to intrinsic physiological limits.

Cetacean biomass, prey consumption, and primary production requirements in the California Current ecosystem

To better understand the role played by cetaceans as top-level predators in the Califor- nia Current ecosystem, we estimate the fraction of annual net primary production (NPP) required to support the prey consumed by cetaceans, using a simple trophic transfer model. The biomass of ceta- cean species in the California Current is calculated as the product of their mean summer and fall abundance during 1991 to 2005 and estimates of mean mass ind. -1 . Total prey consumption by ceta- ceans is estimated from a mass-specific consumption model. NPP is estimated from remote satellite measurements using the Behrenfeld-Falkowski vertically-generalized production model for each of 4 geographic regions. The total biomass of baleen whales exceeds the biomass of toothed whales by a factor of ~2.5; however, the estimated prey consumption by these taxa is nearly equal. Assuming 10% trophic transfer efficiency, cetaceans are estimated to require 32.2 g C m -2 yr -1 of primary pro- duction, or ~12% of the NPP in the study area, to sustain the prey that they directly consume. Because they feed at a lower trophic level, the primary production requirement (PPR) of baleen whales is ~13% of that of toothed whales, despite their 2.5-fold greater biomass. Uncertainty in trophic trans- fer efficiency results in the greatest uncertainty in estimating PPR for these upper trophic predators.

Detectability of penguins in aerial surveys over the pack-ice off Antarctica

Knowledge of penguin abundance at regional and circumpolar scales across the Southern Ocean is important for the development of ecosystem models and to estimate prey consumption by penguins to assess potential competition with fisheries’ operations. One means of estimating penguin abundance is to undertake aerial surveys across the pack-ice surrounding Antarctica where penguins forage. However, it has long been recognised that aerial counts and resultant abundance estimates are likely to be negatively biased unless detectability is estimated and taken into account. Mark–recapture line-transect methods were used to estimate the detectability of penguin groups resting on ice floes during helicopter surveys over the pack-ice off Antarctica. Group size had the greatest effect of several measured covariates on detectability. Despite a concerted effort to meet the central assumption of conventional line-transect sampling (all objects on the transect line are detected), this was close to being achieved by single observers only in the case of the occasional very large group of >20 penguins. Emperor penguins were more detectable than Adélie penguins. Although observers undertook an extensive simulation training program before the survey, overall they improved in their ability to detect penguin groups throughout the survey. Mark–recapture line-transect methods can provide less biased estimation than conventional line-transect methods in aerial survey applications. This improvement comes with some costs, including the need for more demanding data-recording procedures and the need to use larger, more expensive aircraft. These additional costs will often be small compared with the basic cost, but the gain in terms of improved estimation may be substantial.

Dispersal of female southern elephant seals and their prey consumption during the austral summer: relevance to management and oceanographic zones

Summary Numerical models that predict trophic structure require both accurate information on prey consumption rates and estimates of spatial and temporal variation. In the Southern Ocean little information exists on the spatial and temporal patterns of resource use by predators, so we attempted to examine these patterns for an important Antarctic predator, the southern elephant seal. We (i) defined the area of the ocean used by the adult female component of the elephant seal population at Macquarie Island; (ii) quantified the time these seals spent in the different regions of the Southern Ocean; and (iii) estimated the biomass of fish and squid prey consumed per fortnight and per region. We used data from 42 post‐breeding females collected from 1992 to 2001. The data consisted of locations determined by geo‐location (based on light intensity) recorded using dataloggers. A randomized, incremental analysis of at‐sea locations indicated that a sample of 25 individuals was required to provide 95% coverage of the total area of ocean used. The greatest amount of time (44·6%) was spent in the region between the Antarctic Polar Front (APF) and the Subantarctic Front (SAF). Up to 20% of time was spent south of the Antarctic Circle or within Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Statistical Subareas, indicating that seals from Macquarie Island are also important summer‐time predators in high Antarctic waters. The adult female population was estimated to consume 122·73–125·81 × 106 MJ for the post‐lactation foraging trip (31 142–31 925 tonnes of prey). Of this, 47·2–53·4% was consumed within the CCAMLR Statistical Subareas and the Australian and New Zealand exclusive economic zones (EEZ). Synthesis and applications. Our study emphasizes that (i) a large sample of individual seals (25) can estimate spatial trends in prey consumption; (ii) much of the estimated prey consumption occurs within fishery‐managed zones, therefore elephant seals should be included in models predicting trophic structure in the Southern Ocean; and (iii) recent commercial fishery catch within these zones is minimal relative to the prey consumed by elephant seals, but increases in fishing activity in these zones may result in competition for marine resources.

In-depth studies of Magellanic penguin ( Spheniscus magellanicus ) foraging: can we estimate prey consumption by perturbations in the dive profile?

A new concept based on analysis of dive depth data was developed to help estimate prey consumption in ten free-ranging Magellanic penguins (Spheniscus magellanicus) that were brooding chicks. By simultaneously analysing the undulations in the dive depth profile (measured by time-depth recorders, TDRs) and beak opening (obtained from the recently developed intra-mandibular angle sensors, IMASEN), it was possible to determine the proportions of the undulations in the dive profile that resulted (or not) in prey capture. This methodology allowed the number of prey consumed to be estimated with a mean error of 10±6% using TDR data alone. If the mean mass of prey is known, then the overall mass of prey consumed per unit time can be determined. Additionally, the method allows estimation of the depth at which prey is taken and thus indicates how penguins exploit the water column. Due to its simplicity, the proposed methodology has applications for other Spheniscus penguin species and should be considered for other marine endotherm divers that show undulations in the dive depth profile.

Using bioenergetics models to predict stable isotope ratios in fishes

To describe temporal dynamics of stable isotope ratios in fishes, we developed a bioenergetics-based model that links isotope ratios to growth, as influenced by fish size, temperature, diet, and prey quality. The model includes error terms for isotope ratios, diet proportions, and fractionation. The model accurately predicted temporal δ15N dynamics of lake trout (Salvelinus namaycush) in a diet-switch experiment but was less successful for δ13C, possibly because of variable fractionation. The model was then used in three heuristic applications. In a diet-validation scenario, a model derived from limited knowledge of rainbow smelt (Osmerus mordax) diet reasonably estimated δ13C and δ15N compared with a null model but inaccurately estimated prey consumption. In a scenario where adult lake trout briefly cannibalized stocked lake trout fingerlings, the detectability of a cannibalism-induced δ15N increase depended on predator size, duration of cannibalism, and sample size. In a scenario where seasonal isotopic variability occurred at the base of a food web, variation propagated to higher trophic levels depended on consumer size and diet. Our approach is most valuable when used to examine multiple diet combinations that produce observed stable isotope ratios; one can then identify the most reasonable diets through field tests or other observations.

Prey consumption by the burbot (Lota lota) population in Green Bay, Lake Michigan, based on a bioenergetics model

We estimated prey consumption by burbot (Lota lota) based on diet, mortality, growth, maturity, thermal history, population density and a bioenergetics model derived for a similar, cold-water gadoid, the Atlantic cod (Gadus morhua). In Green Bay, Lake Michigan, burbot >400 mm fed primarily on fish; smaller burbot probably fed mostly on invertebrates and sculpins (Cottus sp.). Our calculations indicate that burbot of age ≥1 consumed 16 kg/ha of prey (12.2 kg/ha of fish) in 1988 in the Wisconsin waters of Green Bay including 3.3, 2.1, 1.9, 1.2, and 0.8 kg/ha of rainbow smelt (Osmerus mordax), sculpins, alewife (Alosa pseudoharengus), yellow perch (Perca flavescens), and bloater (Coregnus hoyi), respectively. On an areal basis, piscivory by burbot in Green Bay was higher than the reported lake-wide average for consumption by all salmonids in Lake Michigan. Burbot consumed about 25% of the lake-wide salmonid consumption of alewife per unit area and close to the estimated combined commercial and sport harvest of yellow perch in the Bay the same year (271 vs. 325 tons). Thus, burbot should be included when considering the balance between predatory demand and forage fish production in Green Bay and probably also in other areas of Lake Michigan.

Effect of Sampling Frequency on Intersample Variance and Food Consumption Estimates of Nonpiscivorous Largemouth Bass

We assessed the effect of sampling schedule on estimates of diet composition of an age-II+ cohort of largemouth bass Micropterus salmoides in a small lake in Michiganˈs upper peninsula. Our objectives were to determine the effects of sampling frequency on diet variance and on estimated prey consumption by the fish population. Sampling frequency ranged from 2 to 30 samples per 45-d sampling period in summer 1987. We determined the mean variance of an index of relative importance for each of six prey types as a function of sampling frequency. Estimates of mean rates of total population food consumption (based on percent wet weights of each prey type) were calculated with a generalized bioenergetics model. Feeding on each of three prey items (i.e., zooplankton, chironomids, and trichopterans) fluctuated irregularly over time, and intervals of 3–18 d between feeding bouts were determined from the data. Sampling frequency influenced the accuracy of consumption rate estimates. A minimum sampling frequency of three times per 45 d was necessary for accurate estimates of consumption rates for Chaoborus spp., for odonate naiads, and possibly for ephemeropterans. However, accurate estimates of consumption rates for zooplankton (Daphnia spp. and Holopedium gibberum), trichopterans, and chironomid pupae required more frequent sampling. Increased sampling did not decrease the intersample variance, but the error in estimates of consumption rates decreased with increased sampling frequency.

Limitations of Using Otoliths Recovered in Scats to Estimate Prey Consumption in Seals

Of three length classes of herring, Clupea harengus (30–35, 20–25, and [Formula: see text] total length), fed to a captive harbor seal, Phoca vitulina, otoliths were recovered only from herring 30–35 cm long; of these otoliths only 4% were recovered. Using surface area to volume ratios of otoliths, other fish species were predicted to be similarly underrepresented in scats. After digestion, otoliths showed fewer annuli and a reduction in length and width. Herring otoliths in harbor seal scats indicated the consumption of large herring ([Formula: see text] total length) only and could not be used to estimate numbers of prey consumed. The partial digestion of herring otoliths will result in underestimates of fish age, length, and weight if calculations are based on standard relationships between these parameters and measurements from undigested otoliths.