Cannibals and parasites: conflicting regulators of bimodality in high latitude Arctic char, Salvelinus alpinus

Abstract

Unexploited populations of Arctic char (Salvelinus alpinus) sampled in autonomous lake ecosystems in northern Svalbard demonstrate extraordinary catch curves with age and size frequency distributions characterized by discrete bimodality. Analyses of size‐age relationship, summer diet and food‐related intestinal parasite intensities of modal char groups revealed a pattern of discrete ontogenetic niche shifts. Life‐history changes at age 10–15 and size 200–300 mm/50–300 g involved shifting from an initial mode of small‐sized, slow‐growing and sexually mature individuals feeding on micro‐crustaceans and aquatic insects (Chironomidae, Trichoptera), to a terminal mode of large‐sized and fast‐growing cannibals. Cannibalism, however, was found to result in accumulation of cestodan parasites, of which Diphyllobothrium ditremum increases age‐related mortality rates and may be lethal at 1500–2000 plerocercoids. Genetically allopatric populations with cannibalism demonstrated a female‐biased sex ratio, primarily in the initial mode, suggesting sexual asynchrony in their ontogeny. By contrast, a small population of large‐sized, non‐cannibalistic Arctic char feeding exclusively on the large amphipod Gammaracanthus lacustris, demonstrated unimodal size and age frequency distribution, faster growth, an excess of males and lower parasite burden. Seasonal prey shortage and slow juvenile growth in association with fitness components favoring large body size is a suggested mechanism for inducing cannibalism. Although not the basic cause of bimodality as such, it is concluded that ontogenetic niche shift by cannibalism reinforces discrete age modal divergence resulting in the numerical preponderance of large‐sized individuals in these marginal char populations. Cannibalism is thus considered an important strategy for survival of landlocked Arctic char in the High Arctic. As a conflicting cost to the more efficient use of available energy by larger individuals, the accumulation of cestodan parasites in cannibals, however, will reduce the survival rate of old individuals and accelerate their termination within this modal group.

Seasonal and ontogenetic shifts in the diet of Arctic charr Salvelinus alpinus in a subarctic lake

“…In Saanajärvi S. alpinus, however, the low C:N ratios of both muscle and liver tissue indicated that they contained only a little lipid (unpubl. data).According to the Z estimates, S. alpinus 130-200 mm in Saanajärvi had a more littoral trophic niche than the 200-350 mm fish and did not show a similar Hammar, 2000;Riget et al, 2000;, supporting the argument that in Saanajärvi, the S. alpinus of >200 mm L T have largely outgrown the predation window of cannibals. Such a seasonal shift of intermediate-sized S. alpinus to pelagic feeding during summer or autumn, while the smaller conspecifics remain in the benthic habitat throughout the year, seems to be a rather widespread phenomenon that is often assumed to be due to different predation risks and abilities of individuals of different sizes to exploit seasonally abundant pelagic food resources (Sparholt, 1985;L'Abée-Lund et al, 1993;Bjøru & Sandlund, 1995).…”

“…In allopatric S. alpinus populations, an obligatory shift to piscivory (i.e. cannibalism) occurs at a relatively large size compared with multispecies fish communities, where availability of other small-sized prey fish species enables a shift to piscivory by individuals of L T 100-200 mm and at the population level usually at L T of 200-300 mm (L' Abée-Lund et al, 1992;Hammar, 2000;. The rather late shift to piscivory in Saanajärvi is probably related to the prevailing size structure of the fish population, where any <100 mm fish may be quickly consumed by larger individuals (Polis, 1981;.…”

Persistent organic pollutants in biota samples collected during the Ymer-80 expedition to the Arctic

“…However, in these lakes with cannibalistic populations, during special events, for example, when chironomid or trichopteran pupae hatch, all sizes of Arctic char as well as both terrestrial and marine birds may feed on the insects. During these and other periods of the summer season, presence of bird droppings and remains of marine crustaceans and marine fish in char stomachs indicate a marine source of energy to landlocked Arctic char in the High Arctic (Skreslet 1973;Hammar 2000). A special case is Jensenvatn in which both small and large individuals foraged almost exclusively on the abundant amphipod Gammaracanthus lacustris (Hammar 2000).…”

“…Within some landlocked populations, such as in Annavatn and Wibjørnvatn, char of different size show very different feeding behaviour; small fish that feed mainly on zooplankton and insects grow slowly, while fish of above a certain size turn cannibalistic, which leads to faster growth, larger size, and accumulation of parasites (Hammar 2000). However, in these lakes with cannibalistic populations, during special events, for example, when chironomid or trichopteran pupae hatch, all sizes of Arctic char as well as both terrestrial and marine birds may feed on the insects.…”

Seasonal Variations in the Use of Profundal Habitat among Freshwater Fishes in Lake Norsjø, Southern Norway, and Subsequent Effects on Fish Mercury Concentrations

“…Finstad et al [76]. Another explanation for the differences in the two groups is a dimorphism with invertebrate eating dwarfs and cannibalistic giants [77], which could persist permanently [78]. Parker and Johnson [79], for example, have observed phenological differences between A. charr morphs such as different numbers of gill rakers.…”