J. Brian Dempson

General effects of climate change on Arctic fishes and fish populations.

Abstract Projected shifts in climate forcing variables such as temperature and precipitation are of great relevance to arctic freshwater ecosystems and biota. These will result in many direct and indirect effects upon the ecosystems and fish present therein. Shifts projected for fish populations will range from positive to negative in overall effect, differ among species and also among populations within species depending upon their biology and tolerances, and will be integrated by the fish within their local aquascapes. This results in a wide range of future possibilities for arctic freshwater and diadromous fishes. Owing to a dearth of basic knowledge regarding fish biology and habitat interactions in the north, complicated by scaling issues and uncertainty in future climate projections, only qualitative scenarios can be developed in most cases. This limits preparedness to meet challenges of climate change in the Arctic with respect to fish and fisheries.

An Overview of Effects of Climate Change on Selected Arctic Freshwater and Anadromous Fishes

Abstract Arctic freshwater and diadromous fish species will respond to the various effects of climate change in many ways. For wide-ranging species, many of which are key components of northern aquatic ecosystems and fisheries, there is a large range of possible responses due to inter- and intra-specific variation, differences in the effects of climate drivers within ACIA regions, and differences in drivers among regions. All this diversity, coupled with limited understanding of fish responses to climate parameters generally, permits enumeration only of a range of possible responses which are developed here for selected important fishes. Accordingly, in-depth examination is required of possible effects within species within ACIA regions, as well as comparative studies across regions. Two particularly important species (Arctic char and Atlantic salmon) are examined as case studies to provide background for such studies.

Ecological segregation within and among Arctic char morphotypesin Gander Lake, Newfoundland

SynopsisStable isotopes of carbon and nitrogen were used to examine differences in the feeding ecology of sympatric morphotypes of Arctic char, Salvelinus alpinus, from Gander Lake, Newfoundland. Morphotypes were differentiated on the basis of colour (pale and dark) and meristic characteristics. Significant differences in length, age and habitat depth were also noted, with the pale-form being smaller, younger and caught at greater average depths than the dark-form. Pale and dark morphotypes possessed significantly different carbon and nitrogen stable isotope signatures, with the dark-form being more δ13C enriched than the pale-form. Within each morphotype grouping distinctive isotopic signature differences related to diet (dark-form) and capture depth (pale-form) were also observed. Differences in the isotopic signatures within and among habitat groupings were induced by differences in feeding opportunities arising from differences in habitat use and feeding strategies and generally corroborated by stomach content analysis and an index of dietary overlap. Study results provide evidence for the contention that sympatric forms of Arctic char in Gander Lake occupy significantly different foraging niches within the lake. Results further suggest ecologically relevant separations within morphotype groupings result in lower resource competition.

Spatial and temporal variability in the diet of anadromous Arctic charr, Salvelinus alpinus, in northern Labrador

Spatial and temporal variation in the diet of anadromous Arctic charr, Salvelinus alpinus, was determined from over 2500 prey-containing stomachs collected from three stock complex areas in northern Labrador. Based on the percent wet mass, sand lance, Ammodytes spp., capelin, Mallotus villosus, and sculpins, Triglops spp. and Myoxocephalus spp., were the most important components of the diet in the Voisey stock complex, with capelin, sand lance, and hyperiid amphipods, Parathemisto spp., dominating in the Nain region. In contrast, hyperiid amphipods and sculpins were the main prey items in the more northern Hebron–Saglek complex indicating that diet differs among stocks, and at times over limited spatial scales. The relative importance of prey varied with size of predator. Mean weight of charr has declined over time (p < 0.01), and for one stock complex, is highly correlated with the average quantity of capelin found in the stomachs (r = 0.66, p < 0.001). Dietary shifts observed in anadromous charr in some areas of northern Labrador could be associated, in part, with changes in the distribution of capelin in the northwest Atlantic Ocean, possibly as a result of anomalous ocean climate conditions.

Arctic charr (Salvelinus alpinus): a case study of the importance of understanding biodiversity and taxonomic issues in northern fishes

Charrs, fishes of the genus Salvelinus (Salmonidae), comprise about 7–20+ species important in the ecosystems and fisheries of Arctic and sub-Arctic waters. Charrs vary by life-history type (anadromous vs. lacustrine), are noted for variability in morphology, colouration and feeding and habitat use across their range, and occur from temperate regions to the northernmost areas of land. Charr diversity is manifested as an often confusing array of taxonomic and functional diversity. Components of taxonomic diversity include unresolved species’ complexes, subspecies, glacial lineages and genetic stocks. Features associated with functional diversity include semi-distinct life history variants and differential habitat and feeding tactics resulting in distinctive growth forms. Complex periglacial histories, allopatric and sympatric diversification processes, founder effects, hybridisation and introgression have contributed to the observed variability and complexity. The categorisation of taxonomic and functional diversity and understanding their roles, drivers and vulnerabilities underpins conservation and management efforts for taxa and their ecosystems. In that context, Arctic charr is an excellent example of diversity and the many levels of difficulties associated with determining the significance of diversity. Despite recently increased scientific efforts, resolving the nature and importance of charr diversity remains both a challenge and a priority because rapid anthropogenically driven change, especially climate change, is altering ecosystems and biodiversity faster than diversity can be inventoried, its importance understood and protective measures developed.

Latitudinal variation in growth among Arctic charr in eastern North America: evidence for countergradient variation?

Biological data from 66 populations of Arctic charr, Salvelinus alpinus, from eastern North America were analysed to test the applicability of the countergradient hypothesis as an explanation of differences in seasonally adjusted growth rates. Samples were obtained along a 37° latitudinal gradient and partitioned among anadromous, normal lacustrine, and dwarf lacustrine Arctic charr morphotypes. Models relating length-at-age or age-specific growth rates to latitude were estimated for each morphotype. Length-at-age declined with latitude for anadromous and lacustrine charr. Age-specific growth rates also varied with latitude, particularly for normal lacustrine charr. Results of analyses provide support for the countergradient hypothesis in growth performance of normal lacustrine morphotypes, where northern populations compensate for the shorter growth season with a greater rate of growth than southern populations. Anadromous charr exhibited equivocal evidence of countergradient variation, while results for dwarf lacustrine Arctic charr populations were inconclusive owing to the limited range of ages, and latitudes for which data were available.

A comparison of muscle-and scale-derived δ13C and δ15N across three life-history stages of Atlantic salmon, Salmo salar

Stable isotope signatures were obtained from paired scale and muscle tissue samples from smolt, post-smolt and one-sea-winter adult Atlantic salmon (Salmo salar). Post-smolt and adult scales were separated into central and outer (marine) portions with analyses carried out on the marine growth section of both life-history stages and the central portion for the adult scales. Muscle and scale delta(13)C and delta(15)N signatures were assessed (1) to determine whether a linear relationship exists between tissue types, (2) to determine if a constant offset exists between tissue signatures across all life-history stages, and (3) to evaluate whether underplating imparts a significant bias to life-history scale segments that would preclude their use in retrospective analyses of any ontogenetic dietary changes between life-history stages. Significant correlations were found to exist between muscle and scale stable isotope signatures obtained from smolts (delta(13)C and delta(15)N) and adults (delta(15)N). Both the muscle and the scale signatures captured the dietary shift associated with the transition from freshwater to the marine environment. Post-smolt and adult scales were depleted relative to muscle tissue, which may be attributed to isotopic differences in amino acid composition between muscle and scale tissues. The results suggest that scales may better represent dietary carbon sources because they are not influenced by lipid dynamics. The scale, however, appears less responsive to short-term shifts in diet relative to muscle and, therefore, must be used only to infer seasonally integrated dietary patterns for slow-growing life-history stages.

Survival, Migration Speed and Swimming Depth of Atlantic Salmon Kelts During Sea Entry and Fjord Migration

In contrast to most species of Pacific salmon (Oncorhynchus spp.), Atlantic salmon (Salmo salar L.) is an iteroparous species such that it may survive and return to spawn repeatedly. Little information exists on these survivors (kelts) even though they might contribute significantly to salmon production when returning as repeat spawners. In order to estimate survival, timing of migration, swimming progression and swimming depth of Atlantic salmon kelts during sea entry and fjord migration, 60 individuals were captured, tagged with acoustic transmitters (of which 20 with depth sensors) and released in the River Alta, Northern Norway. In addition, 172 kelts were also tagged with external Carlin tags to obtain reliable recapture rates in the fisheries. Nearly all (95%) kelts tagged with acoustic transmitters were recorded during their outward migration at four transects of acoustic receivers deployed across the river mouth and Alta Fjord. Most of the kelts migrated through the 30 km long fjord in only 1–2 days (mean time 33 h, range 7–138 h) and generally stayed close to the surface during the fjord migration (individual mean depth of 2 m, range of individual means 0–15 m). Ninety-two percent (55/60) were recorded at the outermost transect 30 km from the river mouth, indicating a high minimum survival rate. The high survival rate and fast progression of kelts throughout the fjord indicate that sea-entry and early sea migration is not a critical phase for Atlantic salmon kelts, despite their weakened condition after spawning and overwintering in the river. Even though kelt migration overlapped partly with the fishing season both in the river and the fjord, and the kelts migrated in shallow waters exposed to several types of fishing gear, reported recapture rates were small (3%, 6 of all 232 tagged kelts).

Development of a species-specific fractionation equation for Arctic charr (Salvelinus alpinus (L.)): an experimental approach

A species-specific fractionation equation for Arctic charr (Salvelinus alpinus (L.)) was developed experimentally for use in ecological studies of temperature-driven phenologies for the species. Juvenile Arctic charr were reared in controlled conditions at different temperatures (2–14°C), with three replicates of each temperature. Otoliths from the fish and water samples from the chambers were analysed for oxygen isotope composition and used to estimate temperature-dependent fractionation equations relating the isotopic ratio to rearing temperature. A linear and a second order polynomial relationship were estimated and validated using comparable Arctic charr data from another study. Temperatures predicted using the polynomial equation were not significantly different from recorded experimental temperatures, whereas with the linear equation there were significant differences between the predicted and measured temperatures. The polynomial equation also showed the least bias as measured by mean predictive error. Statistical comparisons of the polynomial fractionation equation to a similarly estimated equation for brook charr (Salvelinus fontinalis (Mitchill)) indicated significant differences. Results imply the need for species-specific fractionation equations, even for closely related fish. Results further suggest the polynomial form of the fractionation equation will facilitate more accurate characterisation of water temperatures suitable for use in ecological studies of temperature-driven phenologies of Arctic charr.

Resilience and stability of north Labrador Arctic charr, Salvelinus alpinus , subject to exploitation and environmental variability

There are relatively few studies that have examined in detail the effects of long term exploitation on population characteristics of Arctic charr, Salvelinus alpinus, coincident with natural changes in environmental conditions. This is particularly the case for anadromous charr despite their widespread use and importance in areas such as northern Canada. Here, the response of north Labrador charr to major reductions in commercial exploitation following long periods of intense fishing is described for three stock complexes with emphasis on changes in size, growth, and age configurations. Changes in age and size distribution, specifically long-term declines in weight, are linked with periods of intensive exploitation and possibly with variability in the climate of the northwest Atlantic that resulted in identifiable diet shifts during the early 1990s. Despite observed changes in some metrics, populations have maintained relatively stable length compositions with variations in weight showing resilience to return to earlier configurations as exploitation was reduced and environmental conditions ameliorated. Results of the long-term investigations challenge traditional beliefs associated with the inability of all anadromous charr to withstand periods of intense exploitation.