Journal of Fish Biology

Electrophoretic studies of five polymorphic enzyme loci (G‐3‐PDH‐2, LDH‐I, LDH‐5, PGI‐2, PGI‐3) in brown trout from Lough Melvin in northwestern Ireland have demonstrated that the morphotypes known by the vernacular names of ‘ferox’, ‘gillaroo’ and ‘sonaghen’, are not merely ecophenotypes but represent genetically distinct and reproductively isolated populations. The results suggest that the long life and higher growth potential of ferox trout of this lake, and possibly others, has a genetic basis. These separate demes of brown trout are probably the result of multiple invasions in post‐glacial times of allopatrically derived stocks. Lough Melvin's isolated position and absence of pike, Esox lucius, and large cyprinids have probably contributed to its pristine condition. As such it is one of the few remaining examples of what may once have been a widespread situation in Britain and Ireland.

Land‐locked populations of Arctic charr in four lakes on Northern Ellesmere Island (80° N) were found to consist of two distinct sizes: ‘dwarf’ and ‘normal’ charr. Both groups attained sexual maturity but differed in appearance and habitat. The smaller fish, occupying the more marginal habitats, retained their parr‐markings; the larger group had the general characteristics of smolts, being more silvery and without parr‐marks. In their juvenile stages, the charr destined to attain the larger group were indistinguishable from members of the smaller group. Although fish in the larger group were capable of cannibalism, this was rarely observed. In general, the fish in the larger group were older than the smaller ones but great variation in size at a given age resulted in certain age classes containing representatives of both groups. The population structure varied considerably between lakes; a high proportion of ‘normal’ charr correlated well with a high growth rate in the first few years of development. It is postulated that the two groups live in dynamic equilibrium where the advantages of progenesis (retention of juvenile characters by adults) in the smaller type are traded against the larger proportion of the energy resources available to the larger type. The concept of heterochrony in an ecological setting is introduced.

The main molecular techniques which can be used to generate genetic markers, and the applications of these markers to studies of fish populations are outlined. Published and ongoing studies, in the authors' laboratories, on brown trout and Atlantic salmon are used to compare the resolution and applicability of allozyme, mitochondrial DNA and minisatellite (variable number of tandem repeats) markers for studies on population structuring, genetic variation within populations, and the impact of the accidental and deliberate introduction of non‐native salmonids on the genetic make‐up of natural populations.

Brown trout in Lake Femund migrated from the nursery streams mainly at 2 years old, but ranging between 1 and 8 years. Brown trout switched to piscivory from 3 years onwards, and a body length of 17·5 cm, according to back calculation from scales. Fast growers switched to piscivory at a younger age and smaller size than slow growers. The most slow‐growing trout switched to fish feeding at 9 years old and a mean body length of 36 cm. The growth of the invertebrate feeders was almost rectilinear to c. 45 cm and 11 years of age. Switching to a fish diet induced increased growth rates. Age at sexual maturity increased with the age at which the fish became piscivorous. The invertebrate feeders matured at an age similar to that of the most fast growing piscivorous trout. The mortality rate of sub‐adults and adults did not differ significantly between fish and invertebrate feeding trout. Longevity of piscivorous trout was estimated at 11 years and of invertebrate feeders at 10 years.

A purified mitochondrial DNA (mtDNA) probe was used to examine restriction fragment length polymorphisms produced by six restriction enzymes (Xba I, Eco RV, Ava II, HinfI, Hae III, Mbo I) in 915 brown trout from western Europe. A total of 20 composite haplotypes were found with one to seven haplotypes in individual populations. Icelandic trout samples from north, south, east, and west coast drainages showed only a single common haplotype in contrast to the high level of polymorphism found in Irish and Scottish populations. The phylogeny of mtDNA haplotypes and the pattern of haplotype distribution suggests that post‐glacial colonization of brown trout in NW Europe was more complex than the dual colonization model which has been proposed on the basis of differential LDH‐5* allele distribution. For example, Lough Melvin (Ireland) appears to have been independently

Size and frequency of occurrence of prey of brown trout Salmo trutta L. and Arctic charr Salvelinus alpinus (L.) were recorded in 13 Norwegian lakes during 1973–1990. Piscivores usually comprised less than 5% of the total population. Arctic charr were less piscivorous than brown trout. Trout and charr became piscivorous at 13 and 16 cm length, respectively. These size thresholds were similar to those of other facultative piscivorous freshwater fish species. When present, three‐spined sticklebacks, Gasterosteus aculeatus (L.), were preferred by all length groups of piscivorous brown trout and Arctic charr. Length of prey increased with increasing predator length, and the mean body length of prey was about 33 and 25% of predator length for trout and charr, respectively. Yearlings of charr were not recorded as prey.

Although the ferox of Scottish Highland lochs (lakes) have long captured the interest of both laymen and scientists, no previous investigation of their biology or ecology has been undertaken. This paper is based on 141 ferox from 22 lochs collected during the last 22 years and the results from a recent investigation into their status and distribution. When the features of their environment and the distribution of what is apparently their main food source, the arctic charr, were investigated, two essential conditions governing the occurrence of ferox emerged and a third appeared to be important: (i) oligotrophic waters; (ii) the presence of charr and (iii) a large loch (over 100 ha in extent). Typically, ferox grow slowly during the first third of life but on reaching what may be a critical length enter into a phase of rapid growth and may eventually reach a size and age very much greater than that of the individuals in the normal trout population from which they arise. This pattern of growth contrasts markedy with that of large, fast growing brown trout from eutrophic waters in Scotland which do not reach the same extremes of age or size.

The Ldh‐5 locus, which codes for the eye‐specific lactate dehydrogenase in brown trout, has been shown to be polymorphic for two codominant alleles, Ldh‐5 (100) and Ldh‐5 (90). The Ldh‐5 (100) allele is present in 11 other salmonid species and is therefore likely to be the ancestral one, whereas the unique brown trout Ldh‐5 (90) allele would seem to be the result of a mutation in that lineage. The Ldh‐5 (90) allele appears to have arisen in north‐west Europe during or after the last glaciation, with allelic substitution taking place under the action of natural selection. The Ldh‐5 (90) allele can be used as a phylogeographic marker to trace the post‐glacial spread of the populations possessing it. Examination of the current distribution of the two alleles suggests that, in the formerly glaciated area of north‐west Europe, there have been two post‐glacial colonizations by brown trout. The first was by an ‘ancestral’ race fixed for the Ldh‐5(100) allele. This was later replaced by, or introgressed with, the later‐arriving ‘modern’ race characterized by the Ldh‐5 (90) allele, except where physical barriers prevented colonization by this latter form. Artificial stocking has resulted in ‘genetic contamination’ of many populations of the ancestral race and there is an urgent need to conserve the remaining pristine populations, especially in view of the likely genetic propensity for longevity and ultimate large size exhibited by this race.

Age, growth and reproductive characteristics of fathead minnow Pimephales promelas populations inhabiting four lakes that varied in the extent and frequency of winterkill were studied in the boreal region of western Canada. The lifespan of fathead minnows inhabiting lakes prone to winterkill was 1–2 years shorter than those in less disturbed lakes. In populations prone to winterkill, fish displayed faster growth rates and grew to a larger size‐at‐age, particularly during the first year of life. Although lower population densities in winterkill lakes probably contributed to this increased growth, adults in these populations tended to spawn earlier in the season than the smaller adults in more stable populations. Fathead minnows in lakes prone to winterkill also matured at an earlier age and allocated a greater proportion of their body mass to gonads than conspecifics in the more benign, stable lakes. These trends are consistent with predictions for organisms in variable, unpredictable environments and, because fathead minnows are tolerant to a wide range of environmental conditions, suggest that variation in life‐history traits among populations is probably a product of both selection and phenotypic plasticity.

Pelagic fish populations in Lake Tanganyika consist mainly of two small clupeid species and four centroponiid species which prey on them. Exploitation with purse‐seines began in the southeast arm in 1962 and catches were sampled up to early 1969. Inshore clupeid populations were also sampled with a scoop‐net (lusenga of the kind used in the traditional inshore fishery.

Clupeid biomass reached annual maxima around September. One clupeid species, Stolothrissa tanganicae Regan, dominated pelagic catches except in 2 years when about equal quantities of the other clupeid, Limnothrissa miodon Blgr. were caught. Numbers of each predator (three species of Lates and Luciolates stappersii Blgr.) decreased from 1963 to 1966 and remained low thereafter. The clupeid catch rose from 1964 to 1967 and remained high in 1968. The average nightly catch weight per year of all species together altered relatively little. Changes in population size distributions occurred.

Sampling methods were concluded to be valid for clupeid populations, but probably indi‐cated only general changes in predator populations. Clupeid life‐cycles are mostly accomplished within a year, and appear closely related to the periodic and spatial variations in plankton production. The two species are competitive and can replace one another in the pelagic zone. Certain distribution patterns of the clupeids and of the predator young are believed to be adaptations to severe predation. Low replacement rates under fishing pressure account for the decline of predator species, and the clupeid increase resulted from reduction in predation. A fairly stable exploited phase has apparently been reached in which clupeid biomass is much greater and predator biomass much less than in the natural state. The data encourage certain predictions. Similar major trends occurred in the much larger fishery in the Burundi sector of the lake.