Thomas R. Hrabik

Diet and Prey Selection by Lake Superior Lake Trout during Spring, 1986–2001

ABSTRACT We describe the diet and prey selectivity of lean (Salvelinus namaycush namaycush) and siscowet lake trout (S. n. siscowet) collected during spring (April–June) from Lake Superior during 1986–2001. We estimated prey selectivity by comparing prey numerical abundance estimates from spring bottom trawl surveys and lake trout diet information in similar areas from spring gill net surveys conducted annually in Lake Superior. Rainbow smelt (Osmerus mordax) was the most common prey and was positively selected by both lean and siscowet lake trout throughout the study. Selection by lean lake trout for coregonine (Coregonus spp.) prey increased after 1991 and corresponded with a slight decrease in selection for rainbow smelt. Siscowet positively selected for rainbow smelt after 1998, a change that was coincident with the decrease in selection for this prey item by lean lake trout. However, diet overlap between lean and siscowet lake trout was not strong and did not change significantly over the study period. Rainbow smelt remains an important prey species for lake trout in Lake Superior despite declines in abundance.

A Synthesis of Cisco Recovery in Lake Superior: Implications for Native Fish Rehabilitation in the Laurentian Great Lakes

Abstract Populations of cisco Coregonus artedi in the Laurentian Great Lakes supported large-scale commercial fisheries and were the primary forage of piscivores during the first half of the 20th century. However, by 1970 populations had collapsed in all of the lakes. Since then, ciscoes have staged a recovery in Lake Superior. In this synthesis, we describe the status of ciscoes in Lake Superior during 1970–2006 and provide a comprehensive review of their ecology. Better understanding of age estimation techniques, application of hydroacoustic and midwater trawl sampling, and compilation of long-term data sets have advanced our understanding of the species. Management agencies contemplating rehabilitation of cisco populations should recognize that (1) knowledge of cisco ecology and population dynamics is increasing; (2) ciscoes are long-lived; (3) Great Lakes populations are probably composed of both shallow-water and deepwater spawning forms; (4) large year-classes can be produced from small adult stocks...

Hydroacoustic estimates of abundance and spatial distribution of pelagic prey fishes in Western Lake Superior

Lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) are a valuable prey resource for the recovering lake trout (Salvelinus namaycush) in Lake Superior. However, prey biomass may be insufficient to support the current predator demand. In August 1997, we assessed the abundance and spatial distribution of pelagic coregonines and rainbow smelt in western Lake Superior by combining a 120 kHz split beam acoustics system with midwater trawls. Coregonines comprised the majority of the midwater trawl catches and the length distributions for trawl caught fish coincided with estimated sizes of acoustic targets. Overall mean pelagic prey fish biomass was 15.56 kg ha−1 with the greatest fish biomass occurring in the Apostle Islands region (27.98 kg ha−1), followed by the Duluth Minnesota region (20.22 kg ha−1), and with the lowest biomass occurring in the open waters of western Lake Superior (9.46 kg ha−1). Biomass estimates from hydroacoustics were typically 2–134 times greater than estimates derived from spring bottom trawl surveys. Prey fish biomass for Lake Superior is about order of magnitude less than acoustic estimates for Lakes Michigan and Ontario. Discrepancies observed between bioenergetics-based estimates of predator consumption of coregonines and earlier coregonine biomass estimates may be accounted for by our hydroacoustic estimates.

Seasonally Dynamic Diel Vertical Migrations of Mysis diluviana, Coregonine Fishes, and Siscowet Lake Trout in the Pelagia of Western Lake Superior

Abstract Diel vertical migrations are common among many aquatic species and are often associated with changing light levels. The underlying mechanisms are generally attributed to optimizing foraging efficiency or growth rates and avoiding predation risk (μ). The objectives of this study were to (1) assess seasonal and interannual changes in vertical migration patterns of three trophic levels in the Lake Superior pelagic food web and (2) examine the mechanisms underlying the observed variability by using models of foraging, growth, and μ. Our results suggest that the opossum shrimp Mysis diluviana, kiyi Coregonus kiyi, and siscowet lake trout Salvelinus namaycush migrate concurrently during each season, but spring migrations are less extensive than summer and fall migrations. In comparison with M. diluviana, kiyis, and siscowets, the migrations by ciscoes C. artedi were not as deep in the water column during the day, regardless of season. Foraging potential and μ probably drive the movement patterns of M. ...

Vertical Distribution of Fish Biomass in Lake Superior: Implications for Day Bottom Trawl Surveys

Abstract Evaluation of the biases in sampling methodology is essential for understanding the limitations of abundance and biomass estimates of fish populations. Estimates from surveys that rely solely on bottom trawls may be particularly vulnerable to bias if pelagic fish are numerous. We evaluated the variability in the vertical distribution of fish biomass during the U.S. Geological Surveys annual spring bottom trawl survey of Lake Superior using concurrent hydroacoustic observations to (1) test the assumption that fish are generally demersal during the day and (2) evaluate the potential for predictive models to improve bottom trawl–determined biomass estimates. Our results indicate that the assumption that fish exhibit demersal behavior during the annual spring bottom trawl survey in Lake Superior is unfounded. Bottom trawl biomass (BBT) estimates (mean ± SE) for species known to exhibit pelagic behavior (cisco Coregonus artedi, bloater C. hoyi, kiyi C. kiyi, and rainbow smelt Osmerus mordax; 3.01 ± 0...

Foraging, bioenergetic and predation constraints on diel vertical migration: field observations and modelling of reverse migration by young-of-the-year herring Clupea harengus

Diel vertical migration (DVM) of young-of-the-year (YOY) herring Clupea harengus and one of their major predators, pikeperch Sander lucioperca, was examined using bottom-mounted hydroacoustics in Himmerfjärden, a brackish bay of the Baltic Sea, in summer. In contrast to previous studies on DVM of C. harengus aggregated across size and age classes, YOY C. harengus showed a reverse DVM trajectory, deeper at night and, on average, shallower during the day. This pattern was observed consistently on five acoustic sampling occasions in 3 years and was corroborated by two out of three trawl surveys. Large acoustic targets (target strength >-33 dB, probably piscivorous S. lucioperca >45 cm) showed a classic DVM trajectory, shallow at night and deeper during the day. Variability in YOY C. harengus vertical distribution peaked at dawn and dusk, and their vertical distribution at midday was distinctly bimodal. This reverse DVM pattern was consistent with bioenergetic model predictions for YOY C. harengus which have rapid gut evacuation rates and do not feed at night. Reverse DVM also resulted in low spatial overlap with predators.

Hydroacoustic Estimation of Zooplankton Biomass at Two Shoal Complexes in the Apostle Islands Region of Lake Superior

ABSTRACT Hydroacoustics can be used to assess zooplankton populations, however, backscatter must be scaled to be biologically meaningful. In this study, we used a general model to correlate site-specific hydroacoustic backscatter with zooplankton dry weight biomass estimated from net tows. The relationship between zooplankton dry weight and backscatter was significant (p < 0.001) and explained 76% of the variability in the dry weight data. We applied this regression to hydroacoustic data collected monthly in 2003 and 2004 at two shoals in the Apostle Island Region of Lake Superior. After applying the regression model to convert hydroacoustic backscatter to zooplankton dry weight biomass, we used geo-statistics to analyze the mean and variance, and ordinary kriging to create spatial zooplankton distribution maps. The mean zooplankton dry weight biomass estimates from plankton net tows and hydroa-coustics were not significantly different (p = 0.19) but the hydroacoustic data had a significantly lower coefficient of variation (p < 0.001). The maps of zooplankton distribution illustrated spatial trends in zooplankton dry weight biomass that were not discernable from the overall means.

A simulation of food-web interactions leading to rainbow smelt Osmerus mordax dominance in Sparkling Lake, Wisconsin

A process-based simulation model was used to examine the nature and intensity of food-web interactions that allow Osmerus mordax to dominate invaded lakes. The model simulates food-web interactions among linked populations of O. mordax, Coregonus artedi and Sander vitreus. Simulations indicated that O. mordax dominate where: (1) adult O. mordax prey on young-of-the-year (YOY) C. artedi, (2) YOY O. mordax negatively affect YOY S. vitreus through competition and (3) adult S. vitreus experience moderate fishing mortality. Osmerus mordax dominated simulations across a broad range of variable values that regulated competition and predation, and displayed threshold responses to increasing angler harvest. Consequently, angler harvest should be carefully managed in lakes susceptible to O. mordax invasions because the alternative could lead to fishery collapse.

Channel Morphology Response to Selective Wood Removals in a Sand-Laden Wisconsin Trout Stream

Abstract Large sand bed loads in trout stream headwaters can limit salmonid spawning habitat and reproductive success. This phenomenon has been observed in many northern Wisconsin watersheds, where historic logging practices are the likely source of the sediment loading. Presently, sediment transport is limited by abundant woody debris, causing channels to aggrade and bury gravels. We evaluated the impacts of a wood debris and beaver dam removal remediation strategy on fine sediment transport and exposure of the underlying gravel and cobble substrates in a second-order Lake Superior tributary. A 300-m treatment reach received selective wood removals and was compared with both an upstream 300-m reference reach receiving no alteration and a downstream reach to monitor the effects of transferred fine sediment. Physical channel measurements were taken before the wood removal process, with repeat sampling at 10 and 12 months posttreatment. The wood removal treatment resulted in a significant 25% narrowing of m...

Densities and diel vertical migration of Mysis relicta in Lake Superior: a comparison of optical plankton counter and net-based approaches

The opossum shrimp, Mysis relicta, is an important prey item for benthic (e.g., deepwater sculpin, Myoxocephalus thompsoni) and pelagic (e.g., kiyi, Coregonus kiyi, bloater, Coregonus hoyi, and rainbow smelt, Osmerus mordax) fishes in Lake Superior (Anderson & Smith 1971, Selgeby 1988, Johnson et al. 2004). It is also an important consumer of zooplankton and phytoplankton (Bowers & Grossnickle 1978, Johannsson et al. 1994, Gal et al. 2006). Mysis is known to exhibit classic diel vertical migration (DVM; Hutchinson 1967) in Lake Superior (Bowers 1988) and other lakes (Beeton 1960, Beeton & Bowers 1982), with the majority of the population located high in the water column at night and in deep water, near or on the bottom during the day. The DVM of Mysis appears to be an important, but not exclusive, driver of DVM patterns in coregonids (Eshenroder & Burnham-Curtis 1999, Hrabik et al. 2006, Jensen et al. 2006). Despite the ecological importance of Mysis, the last published estimate of their densities and vertical distribution in Lake Superior was based on vertical net tows conducted at a single location in 1986 (Bowers 1988). Optical plankton counters (OPCs) use a beam of light to count and measure particles passing through an aperture (Herman 1992). They have been used to estimate Mysis density in Lake Ontario (Gal et al. 1999a, Sprules 2000) and zooplankton biomass and size spectra in several of the Great Lakes (Stockwell & Sprules 1995, Stockwell et al. 2002, Yurista et al. 2005, 2006). The newer laser OPC (LOPC) uses a narrow laser beam that allows better performance in high particle densities and direct estimation of flow speed through the aperture by timing the passage of small particles (Herman et al. 2004). It is also capable of generating an image or “shape profile” of larger particles. In this study, we used data from an OPC, an LOPC, and vertical net tows to estimate densities and describe the day/ night vertical distribution of Mysis at a series of stations distributed throughout Lake Superior, and to evaluate the efficacy of using OPC and LOPC for examining DVM of Mysis.