Ontogenetic development of the endangered Atlantic whitefish (Coregonus huntsmani Scott, 1987) eggs, embryos, larvae, and juveniles

Atlantic whitefish ( Coregonus huntsmani Scott, 1987 ) are an endangered species endemic to Canada and are known only from Nova Scotia. Information concerning their early life-history stages — not previously described — is required to support several recovery actions under the federal Species at Risk Act (SARA). We describe the ontogenetic development of Atlantic whitefish eggs, larvae, and juveniles acquired through captive-mating of wild-caught adults, following criteria established for fishes in general and for other North American coregonine species where possible. We then conduct interspecific comparisons between Atlantic whitefish and the two most closely phylogenetically related species (i.e., lake whitefish ( Coregonus clupeaformis (Mitchill, 1818)) and cisco ( Coregonus artedii Lesueur, 1818)) to (i) identify ontogenetic differences that may facilitate field identification, (ii) determine whether differences at common ontogenetic milestones reinforce the recognition of Atlantic whitefish at the species level, and (iii) comment on whether these differences are consistent with the putative ancestral status of Atlantic whitefish. Interspecific differences were observed at all ontogenetic stages, several of which may serve as field identification criteria. Evidence for heterochrony during embryogenesis, as well as interspecific differences in egg size, myomere counts, and pigmentation patterns, substantiate the recognition of Atlantic whitefish as a valid species and support its putative ancestral status.

Changes in zooplankton and the phenology of the spiny water flea, Bythotrephes, following its invasion of Harp Lake, Ontario, Canada

The crustacean zooplankton community of Harp Lake, Ontario, Canada, has changed appreciably since the invasion by the spiny water flea, Bythotrephes. Crustacean species richness has declined, large-bodied Cladocera have replaced small-bodied ones, and there has been a downward trend in the total abundance of zooplankton because copepod abundance has remained stable while Cladoceran abundance has declined. Although the zooplankton community has now been stable for 4 years (1995–1998), the biology of the invader has changed dramatically. In particular, there have been 10-fold differences in the mean annual abundance of Bythotrephes in this 5-year period and substantial changes in the timing of population maxima. We attribute these changes to two factors: (i) transition from a summer to a fall switch from parthenogenesis to gametogenesis and (ii) interannual differences in the thickness of a warm, dark stratum in the lake. We hypothesize that this stratum provides a refuge for Bythotrephes from predation by lake herring, Coregonus artedii.

Predicting the effects of rainbow smelt on native fishes in small lakes: evidence from long-term research on two lakes

We assessed predictability of negative interactions between native fishes and exotic rainbow smelt (Osmerus mordax) through field experiments and long-term data analysis for two lakes in Wisconsin. Predictions were made based on thermal preferences, diet characteristics, and published accounts of interactions between smelt and yellow perch (Perca flavescens) and smelt and cisco (Coregonus artedii). Our results indicate predation is the most likely cause for the extirpation of cisco from Sparkling Lake in 1990. In Crystal Lake, native yellow perch experienced significant overlap in distribution and diet with smelt. The condition of adult perch was negatively correlated with smelt abundance indicating competition was occurring. Smelt feed on a wide size range of prey items making this species a threat to native fishes, especially when spatial overlap is high. Information on spatial and temporal overlap and diet enable useful predictions about the effect of smelt invasions on native fishes.

Changes in fish populations affected by the construction of the La Grande complex (Phase I), James Bay region, Quebec

Catches per unit of effort (CPUE) with experimental gill nets, recruitment, growth, and condition were monitored between 1977 and 1992 to evaluate the impact of impoundment on the main fish species of La Grande 2, Opinaca, and Caniapiscau reservoirs and the Boyd–Sakami diversion. CPUE and recruitment of northern pike (Esox lucius) and lake whitefish (Coregonus clupeaformis) increased markedly at most stations after impoundment and decreased at the end of the series. The lake whitefish and cisco (Coregonus artedii) showed their most striking rise in CPUE at two bay stations of La Grande 2 and Opinaca reservoirs. CPUE and recruitment of the longnose sucker (Catostomus catostomus), white sucker (Catostomus commersoni), and lake trout (Salvelinus namaycush) (Caniapiscau) showed a general decrease following impoundment. CPUE for the walleye (Stizostedion vitreum) also decreased at several stations; however, the two most southerly stations in La Grande 2 reservoir and the Boyd–Sakami station showed high CPUE during the series. Concentration–redistribution phenomena explain part of the observed variations in CPUE. Correlation analyses showed that walleyes and white suckers were attracted to the warmer, more turbid stations, and that the high primary and secondary productivity of bay stations attracted the coregonines. Growth and condition of the main species increased during variable time intervals after impoundment and decreased at the end of the series.

Early life stages, growth, and reproduction of spring-spawning ciscoes (Coregonus artedii) in Lac des Écorces, Quebec

The biological characteristics of an allopatric population of spring-spawning ciscoes, an extremely rare ecotype, were studied at Lac des Écorces (southwestern Quebec). Environmental conditions during the early life stages are marginal for the species; the eggs are deposited in the deeper section (>20 m) of the lake and the first larva was caught on 30 July. Epilimnetic temperatures were then near the upper lethal threshold for fall cisco larvae. Lac des Écorces ciscoes are very small at the end of their 1st year of life (mean total length 58 mm). They more than double in length during their 2nd year (mean total length 161 mm). Growth thereafter, longevity, fecundity, and egg diameter are within the observed ranges for the species. Spawning occurs from mid-May to early June. Males and females spawn for the first time at age 3. The best predictor of fecundity is liver weight. Gametogenesis occurs under environmental conditions different from those encountered by fall-spawning salmonids. Seasonal changes in the gonadosomatic and hepatosomatic indexes are discussed in terms of temperature, photoperiod, and hormonal influences.