Thomas A. Johnston

Maternal influences on population dynamics: evidence from an exploited freshwater fish

We used a field experiment, population modeling, and an analysis of 30 years of data from walleye (Sander vitreus; a freshwater fish) in Lake Erie to show that maternal influences on offspring survival can affect population dynamics. We first demonstrate experimentally that the survival of juvenile walleye increases with egg size (and, to a lesser degree, female energy reserves). Because egg size in this species tends to increase with maternal age, we then model these maternal influences on offspring survival as a function of maternal age to show that adult age structure can affect the maximum rate at which a population can produce new adults. Consistent with this hypothesis, we present empirical evidence that the maximum reproductive rate of an exploited population of walleye was approximately twice as high when older females were abundant as compared to when they were relatively scarce. Taken together, these results indicate that age- or size-based maternal influences on offspring survival can be an important mechanism driving population dynamics and that exploited populations could benefit from management strategies that protect, rather than target, reproductively valuable individuals.

Relationships Between Body Condition Indices and Proximate Composition in Adult Walleyes

Abstract Relationships between indices of condition and measures of proximate body composition were examined among and within nine populations of mature walleyes Sander vitreus that varied widely in growth rate and lipid status. Body condition, measured as the residuals from mass versus length regressions, was negatively related to mean water content and positively related to mean lipid content among populations. When the roles of water and lipid content were examined simultaneously within populations, most of the variance in condition was accounted for by lipid content. The strength of the relationships between various condition indices and body proximate composition varied considerably among populations but less between sexes within populations. Furthermore, the strength and nature of these relationships was not strongly influenced by the choice of mass-at-length index (residuals, relative condition, relative weight, or Fultons K). The visceral-somatic index and muscle lipid content both exhibited slig...

Maternal Transfer of Organochlorines to Eggs of Walleye (Stizostedion vitreum) in Lake Manitoba and Western Lake Superior

Abstract Gravid walleye were sampled from Lake Manitoba and western Lake Superior (St. Louis River) to measure the concentrations of organochlorine contaminants (OCs) in eggs and muscle and to assess the influence of maternal age and size on the transfer of OCs from mother to egg. Concentrations of most OCs in Lake Superior walleye eggs were 1 to 3 orders of magnitude greater than in Lake Manitoba eggs. Toxaphene (mean concentration (wet weight) ± 1 SE, 1580 ± 462 ng/g) and polychlorinated biphenyls (PCBs) (240 ± 24 ng/g) were the predominant OCs in Lake Superior walleye eggs, whereas DDT and metabolites (eggs 16 ± 1.5 ng/g, muscle 2.1 ± 0.36 ng/g) and PCBs (eggs 9.2 ± 0.83 ng/g, muscle 2.0 ± 2.4 ng/g) were the most common OCs in Lake Manitoba walleye eggs and female muscle. Egg size (dry mass) and the concentration of most OCs in Lake Manitoba walleye eggs were positively correlated with female length and age. This relationship was strongest for more hydrophobic OCs (e.g., PCBs) but was not significant for less hydrophobic OCs (e.g., hexachlorocyclohexanes (HCHs)). Neither egg size nor egg OC concentration of Lake Superior walleye were significantly correlated with female length or age. There was no relationship between OC concentrations in muscle tissue and female length or age of Lake Manitoba walleye. OC concentrations in Lake Manitoba walleye eggs were not correlated with concentrations in the muscle tissue of the mothers, suggesting that OCs in walleye eggs are derived from various tissues. A positive relationship between the egg:muscle ratio of PCB concentrations and the egg:muscle ratio oflipid in freshwater fish suggests that the maternal transfer of PCBs in freshwater fish is related to the relative amounts of lipid in the eggs and mother. The transfer of hydrophobic OCs from mother to eggs in freshwater fish appears to vary within and among fish species and with the hydrophobicity of the OC.

Response of Native and Naturalized Fish to Salmonid Cage Culture Farms in Northern Lake Huron, Canada

Abstract Cage culture of salmonids has expanded into the northern Great Lakes in recent decades, but the effects of this industry on native and naturalized freshwater fish communities have yet to be fully explored. We compared the relative abundance, community composition, and stable isotope composition of wild fish and selected macroinvertebrates between cage and reference sites in the Manitoulin Island region of northern Lake Huron to test the proposition that cage culture operations have altered their distributions and trophic ecologies. Catch per unit effort was higher at cage sites than at reference sites for both small- and large-bodied fishes, but not for crayfish. Species composition varied significantly between cage and reference sites for the small-fish assemblage but not for the large-fish assemblage. The stable isotope ratios (δ13C, δ15N, and δ34S) of dreissenid mussels, crayfish (family Cambaridae), and four species of small-bodied fish did not differ greatly between the cage and reference si...

Stability in Life History Characteristics among Burbot Populations across Environmental Gradients

Abstract We examined the variation in the growth, morphometry, life history, and reproductive traits of 24 lake-dwelling populations of Burbot Lota lota from across Canada with respect to latitude and lake characteristics as well as the differences in these traits between the sexes across populations. Overall, there was stability in most of the life history traits examined vis-a-vis the environmental gradients tested (latitude, Secchi depth, lake area, and maximum lake depth). Among populations, spawning dates ranged from the last week of January to mid-March (most commonly within the first three weeks of February), and date of spawning was not strongly related to latitude or lake characteristics. Liver size and gonad size did not vary significantly with latitude or other lake characteristics, and neither gonad size nor egg size were strongly related to maternal size. However, egg size decreased with latitude and increased with lake area. The proportion of females in nonspawning condition within populatio...

Temporal changes in mercury concentrations of large-bodied fishes in the boreal shield ecoregion of northern Ontario, Canada

Much of the mercury (Hg) in freshwater fish of the boreal shield ecoregion is believed to originate from atmospheric deposition. As such, declines in fish Hg concentrations would be expected in response to recent declines in atmospheric Hg deposition in this ecoregion. We compared recent (2005-2010) and historic (1974-1981) muscle total mercury concentrations ([THg], standardized to a fish body mass of 1 kg) in seven fish species (five piscivores, two benthivores) from 73 lakes in northern Ontario (Canada) using a paired-comparisons approach. The rate of bioaccumulation (i.e., slopes of log(e)[THg] vs log(e) total length relationship) increased for walleye (Sander vitreus) but did not change significantly for any other species. There was no significant decline in mean [THg] between recent and historic time periods for any species. In fact, recent mean [THg] were slightly higher (<0.08 ppm) than historic mean [THg] for all species, and this difference was significant for northern pike (Esox lucius). The magnitude of the temporal change in northern pike declined significantly from south to north over the study area but there were no discernible geographic patterns in the temporal change in [THg] for any other species. This study shows that [THg] of most large-bodied fish species in boreal shield lakes are not declining in response to the decline in atmospheric Hg deposition.

Hearing Sensitivity of the Burbot

Abstract Acoustic communication is central to the reproductive ecology of many fish species, particularly when conditions prevent the use of visual mating cues. The Burbot Lota lota is a freshwater codfish that spawns in a light-limited, under-ice environment. Both sexes possess swim bladder muscles, suggesting that both sexes engage in vocalization and that auditory cues are important to their mating system, but research on acoustic communication has been very limited in this species. In the current study we assessed the hearing sensitivity of Burbot from different size-classes. Burbot hearing was found to be more sensitive in juveniles than in adults, but across size-classes it was most sensitive at lower frequencies, which is similar to results with other codfishes and corresponds to the sounds produced by gadoids. Anthropogenic noise has the potential to disturb fish. The information gained in this study can be useful in assessing the impact of such noise, particularly under ice cover when Burbot are ...

Optical Properties of Dissolved Organic Matter and Their Relation to Mercury Concentrations in Water and Biota Across a Remote Freshwater Drainage Basin

Dissolved organic matter (DOM) includes an array of carbon-based compounds that vary in size and structure and have complex interactions with mercury (Hg) cycling in aquatic systems. While many studies have examined the relationship between dissolved organic carbon concentrations ([DOC]) and methyl Hg bioaccumulation, few studies have considered the effects of DOM composition (e.g., protein-content, aromaticity). The goal of this study was to explore the relationships between total and methyl [Hg] in water, invertebrates, and fish and optically derived measures of DOM composition from 47 lake and river sites across a boreal watershed. Results showed higher aqueous total [Hg] in systems with more aromatic DOM and higher [DOC], potentially due to enhanced transport from upstream or riparian areas. Methyl [Hg] in biota were all positively related to the amount of microbial-based DOM and, in some cases, to the proportions of labile and protein-like DOM. These results suggest that increased Hg bioaccumulation is related to the availability of labile DOM, potentially due to enhanced Hg methylation. DOM composition explained 68% and 54% more variability in [Hg] in surface waters and large-bodied fish, respectively, than [DOC] alone. These results show that optical measures of DOM characteristics are a valuable tool for understanding DOM-Hg biogeochemistry.

Assessing disturbance from under-ice noise on fishes in boreal lakes.

There are many potential sources of anthropogenic noise that can manifest under ice in boreal lakes that are within the hearing ranges of northern boreal fishes. Impacts of noise on fish can be correlated to the fishs hearing sensitivity. In general, boreal fishes have most sensitive hearing <400 Hz,but this varies by species and life stage. By assessing the hearing capabilities and vocalizations of fish species and sound signatures from development activities, it may be possible to forecast potential impacts resulting from expected under-ice noise exposure.There is still the need for basic information to facilitate the assessment and identification of potential impacts (e.g., the hearing capabilities of many species of boreal fishes at different life stages and the sound signatures of various anthropogenic noise sources). Additionally, the impacts to fish from a particular noise source (e.g., pile driving) or the potential for a sound-related disturbance ata particular time in a species life history, such as courtship or spawning, warrants further study.

Maternal influences on thiamine status of walleye Sander vitreus ova

Concentrations of the various forms of thiamine (vitamin B(1) ) were determined in walleye Sander vitreus ova from three central North American lakes. Total thiamine concentrations in ova from Lake Winnipeg S. vitreus were approximately three times greater (mean 12 nmol g(-1) ) than in those from Lakes Erie or Ontario. The percentage of thiamine in the active form (thiamine pyrophosphate, TPP) was highest in Lake Ontario ova (mean 88%) and lowest in those from Lake Winnipeg (mean 70%). Neither ova total thiamine concentration nor per cent ova thiamine as TPP showed any consistent relationships with maternal age, size, morphometric condition, somatic lipid concentrations or liver lipid concentrations. Ova total thiamine concentration, however, was negatively related to ovum size in some populations, as well as among populations, and was positively related to liver total thiamine concentration. Maternal transfer of thiamine to ova appears to be independent of female ontogenetic or conditional state in S. vitreus.