Great variations were observed in length-at-age among populations of white sucker, Catostomus commersoni, in 32 Ontario lakes. The spatial structuring of these data was examined using the Mantel test with respect to the corresponding spatial structuring for various environmental variables which might affect growth of the white sucker. These environmental variables include lake geographic location, lake morphometry, water chemistry, food supply, presence/absence of major predator species, and population density and length at sexual maturity of the white sucker. Geographic distances among lakes and among-lake differences in water chemistry were significantly related to among-population differences in length-at-age. Among-lake differences in lake morphometric variables, benthos densities, presence/absence of predator species, and length at maturity and population density of the white sucker were not significantly related to among-population differences in length-at-age. No sex-specific differences in the effects of environmental variables on length-at-age were observed. This study suggests that the among-lake differences in water chemistry (thus, physiological stresses) and isolation-by-distance (thus, genetic forces) are the two most important factors in patterning the large variations in length-at-age among white sucker populations. However, discerning the separate effect of each of these two factors is not possible because the spatial patterns of these two factors are related.

Onchocerciasis is a widespread disease in intertropical Africa, which, ultimately, causes irreversible blindness. The disease is transmitted by a small blackfly, Simulium damnosum (Diptera), which has aquatic larval and pupal stages. The breeding sites of the blackflies are riffles. These river reaches are the targets of the control campaign of the Onchocerciasis Control Programme in West Africa (OCP). An aquatic monitoring network covering the totality of the area exposed to the insecticide was set up to evaluate environmental impact. In this paper, we present results from the OCP 20-year period of monitoring of the ichthyofauna regularly exposed to larvicides. We do not record any measurable effects of pesticides on the CPUE, abundance of species, trophic structure, community structure or fish health. However, we detect the emergence of a number of medium-term tendencies. These tendencies may relate to climatic conditions that have a consequent effect on hydrology. Thus, we note a constant decrease in the CPUE from the beginning of the monitoring until 1995. The rivers were treated during that time until 1990 or 1993, depending on the station. But even after the treatments ended, the number of catches continued to decrease. As a result, we consider other factors to be the cause of that decline. The average level of annual discharges in this region has been decreasing regularly from the beginning of the 1970s. The production of fish fluctuates in all the rivers according to the flood rate. Important floods inundate larger areas, making greater quantities of food available, and thus improving the conditions for reproduction. The determining factor of the ichthyological stock abundance seems to depend both on the extent and the duration of the flood. In our catches, the observed effect was not immediately evident but appeared a few years later as a cumulative effect of poor hydrological conditions. An increase in the CPUE since 1996 has been related to improved hydrological conditions. In these last few years, we have observed an intensification of the basic flow leading to a ground water renewal. Furthermore, on three of the stations investigated, it appeared that the impediment of rivers (dams) could induce different and/or antagonistic effects. In some cases, we have observed that in spite of unfavourable hydrological conditions, certain species appear to be favoured by the presence of the dam. But, the damming of the river has a negative effect on other species, particularly on the coefficient of condition of migratory fishes. The impact of these factors is enhanced by the fact they exist conjointly.

Fish catches around the Great Lake, Tonle Sap river and the transitional zone between the lake and the river were studied by professional ‘fishing lot' (i.e. fishing zone) from 1995 to 1997. One hundred and twenty species of fish, belonging to 26 families and nine different orders were recorded. Compared to previous studies (1936–1976), only 53 % of families, 32 % of genera and 54 % of species were collected by professional fishing lot. This important loss in biodiversity could have several causes: bias of sampling procedures between professional fishing and research sampling, overfishing, modification of the floodplain by deforestation, etc. The professional fishing data showed that Cypriniformes accounted for 41.6 % of the total number of individuals caught, the Cyprinidae family alone represented 40 species. Siluriformes made up 21.6 %, Perciformes 13.3 %, with six other orders represented by smaller numbers. Eighty-seven percent of the recorded species occur in both habitats depending on the season. Migration takes place from the Mekong river to the Great Lake through the Tonle Sap river at the beginning of the rainy season (May–October), and in the reverse direction at the start of the dry season (November–February). The majority of the species reproduce at the start of the rainy season (May–June) in the flooded plain and the forest floodplain of the Great Lake; the period, the place and the means of reproduction have not been closely studied, particularly for the Belontiformes, Clupeiformes, Synbranchiformes, Pleuronectiformes and Tetraodontiformes. Multivariate analysis of fishing data (November–February) shows three distinct communities: that of the lake (Perciformes and Siluriformes), of the river (Pleuronectiformes, Cypriniformes, Clupeiformes and Siluriformes), and of the transitional zone formed from the principal channel, the old river channel and the oxbow lakes (Cypriniformes, Siluriformes and Osteoglossiformes).

Signal crayfish (Pacifastacus leniusculus) originating from Lake Tahoe, California, were introduced into Karisjärvi, a small (11 ha) lake in central Finland (61° 58'N, 25° 32'E), in 1968 and 1969. Since then, the population has been monitored regularly by trap catches. The stocked signals (3–5 years old) were caught until 1973, by which time they had reached ages of 7–9 years. Catches have increased slowly since the early 1980s, the peak occurring in 1991 at 2.0 individuals per trap per night. The population size was estimated as 60 trappable specimens in 1974, 95 in 1981 and 420 in 1988. The mean density of adult population in suitable biotopes was low, 0.07 specimens per m–2, or 0.3 per shore metre, in 1988. The slow development of the P. leniusculus population has been attributed to environmental factors, mainly the limited area of good crayfish habitat. More than 90 % of all signal crayfish were caught in one-third of the shore area, i.e. in steeply sloping lake beds suitable for burrowing or rich in shelters such as rocks and submerged trees. The signals avoided flat, soft bottoms. Fifty per cent of the female P. leniusculus matured at 90 mm TL (smallest 64 mm), i.e. half of the females entered the breeding population in the autumn of their fourth year. The mean size of newly hatched (stage 2) juveniles was 9.7 mm TL and of one-summer olds 30.3 mm. The largest specimen trapped measured 159 mm. The signal crayfish imported into Finland and stocked were infected with crayfish plague (Aphanomyces astaci), but no mortality has been recorded. Two Branchiobdellidae (Annelida, oligochaeta) epibiont species new to Finland were imported from North America with P. leniusculus. The continuous occurrence of these commensals in the signal crayfish population indicates that they have adapted to Finnish conditions.