West Coast River Research Articles

PREDICTING FISH HABITAT USE TO CHANGES IN WATER FLOW: MODELLING CRITICAL MINIMUM FLOWS FOR ATLANTIC SALMON,SALMO SALAR, AND BROWN TROUT,S. TRUTTA

To identify possible critical minimum flows, habitat availability and habitat selection by young Atlantic salmon and brown trout on selected stream reaches were studied in a spatially and temporally heterogeneous Norwegian west coast river. Based on direct underwater observations of 1768 individuals across transects on each reach, unimodal habitat suitability curves for water depth, water velocity and substrate were produced for both species. There was considerable spatial niche overlap between the two species, suggesting niche competition. Rather than narrow optima, they selected habitats within broad ranges of values of the measured microhabitat variables. Atlantic salmon used the broader range. Spatial variation in habitat use was attributed to different habitat availabilities, whereas temporal variations were partly explained by varying water flows and temperature. Variation in hydrophysical conditions, i.e. habitat availability, was quantified by measuring in the same transects at different water flows. A new hydraulic simulation model, the RIMOS (RIver MOdelling System), was developed to allow for hydraulic simulation of habitat availability at a spatial and temporal scale relevant to the fish. By comparing spatial variations in habitat use and availability, habitat preference curves were calculated and applied in the model. Modelling of the availability of suitable habitats from extreme summer minimum flows to 30 year high flows identified critical minimum flow levels, below which suitable habitat was drastically reduced. These values depended on the habitat variable, fish species, temperature and spatial heterogeneity in the habitat. The negative effects of reduced water flow on fish habitat suitabilities were most pronounced for water velocities and for Atlantic salmon. Thus through hydraulic modelling combined with fish habitat preference data, it was possible to predict a likely change in species composition. Habitat–hydraulic modelling also indicated that resilience towards reduced water flows depended on the in situ stream structure and was much greater in pool-like stream reaches than in riffle/rapids areas. Modelling also allowed for time series analysis of fish habitat suitability to identify temporal bottlenecks in available spatial niches. Furthermore, some exploratory modelling was carried out to identify landscape phenomena in fish habitat use. Habitats were considerably more fragmented in riffle/rapids habitats than in pools, where three-dimensional connectivity tended to be complete over a wider range of flows. It is concluded that fish habitat selection data combined with hydraulic modelling at a scale relevant to the fish can be a useful tool in stream management. However, there is no such a thing as ‘the’ suitable minimum flow; the effect of reduced flows will vary with stream structure, the hydro-physical variables in question and the fish species. More studies are needed to elucidate possible spatial and in particular temporal variations in fish habitat selection. Care must be taken in aggregating habitat suitability data into single-valued functions.

PREDICTING FISH HABITAT USE TO CHANGES IN WATER FLOW: MODELLING CRITICAL MINIMUM FLOWS FOR ATLANTIC SALMON, SALMO SALAR, AND BROWN TROUT, S. TRUTTA

To identify possible critical minimum flows, habitat availability and habitat selection by young Atlantic salmon and brown trout on selected stream reaches were studied in a spatially and temporally heterogeneous Norwegian west coast river. Based on direct underwater observations of 1768 individuals across transects on each reach, unimodal habitat suitability curves for water depth, water velocity and substrate were produced for both species. There was considerable spatial niche overlap between the two species, suggesting niche competition. Rather than narrow optima, they selected habitats within broad ranges of values of the measured microhabitat variables. Atlantic salmon used the broader range. Spatial variation in habitat use was attributed to different habitat availabilities, whereas temporal variations were partly explained by varying water flows and temperature. Variation in hydrophysical conditions, i.e. habitat availability, was quantified by measuring in the same transects at different water flows. A new hydraulic simulation model, the RIMOS (RIver MOdelling System), was developed to allow for hydraulic simulation of habitat availability at a spatial and temporal scale relevant to the fish. By comparing spatial variations in habitat use and availability, habitat preference curves were calculated and applied in the model. Modelling of the availability of suitable habitats from extreme summer minimum flows to 30 year high flows identified critical minimum flow levels, below which suitable habitat was drastically reduced. These values depended on the habitat variable, fish species, temperature and spatial heterogeneity in the habitat. The negative effects of reduced water flow on fish habitat suitabilities were most pronounced for water velocities and for Atlantic salmon. Thus through hydraulic modelling combined with fish habitat preference data, it was possible to predict a likely change in species composition. Habitat–hydraulic modelling also indicated that resilience towards reduced water flows depended on the in situ stream structure and was much greater in pool-like stream reaches than in riffle/rapids areas. Modelling also allowed for time series analysis of fish habitat suitability to identify temporal bottlenecks in available spatial niches. Furthermore, some exploratory modelling was carried out to identify landscape phenomena in fish habitat use. Habitats were considerably more fragmented in riffle/rapids habitats than in pools, where three-dimensional connectivity tended to be complete over a wider range of flows. It is concluded that fish habitat selection data combined with hydraulic modelling at a scale relevant to the fish can be a useful tool in stream management. However, there is no such a thing as ‘the’ suitable minimum flow; the effect of reduced flows will vary with stream structure, the hydro-physical variables in question and the fish species. More studies are needed to elucidate possible spatial and in particular temporal variations in fish habitat selection. Care must be taken in aggregating habitat suitability data into single-valued functions.

Diseases of wild sea trout in Scotland in relation to fish population decline

Sea trout cathceshave been unusually low in recent years in some areas of Scotland, particularly parts of the west coast, coincident with a decline in sea trout catches and stocks in parts of western Ireland. The causes of the widespread decline are at present unknown and are being investigated but climatic as well as biological influences are likely to be involved. A wide variety of pathologies has been detected in sampled sea trout, particularly in the kidney, liver, and heart but with no overall consistency in occurrence between areas or within individual rivers. In some fish the lesions were sufficiently extensive to be considered to be possibly threatening to organ function, but no correlation was established between severity and loss of fish condition. Juvenile trout sampled in seawater, in estuaries and lower rivers were mainly in good condition. However, some obtained in rivers in northwestern Scotland during early summer had cranial lesions and dorsal fin damage associated with sea lice. The prevalence and intensity of infection varied between and within sites with high intensities being found on both west and east coast rivers. There was no clear relationship between lice burden and the extent of fin damage on individual fish. Samples obtained in the late summer and autumn were clear of lice and showed no lesions. A wide range of other metazoan parasites was recorded, with no indication of correlation with host condition. No significant bacterial infection was detected, but infectious pancreatic necrosis (IPN) virus was detected in fish from two west coast rivers. The significance of the pathologies and occurrence of metazoan parasite and other infections is being studied but with the data currently available no cause-effect relationship between disease and decline of sea trout populations could be established.

Effects of Oceanic Variations and the West Greenland Fishery on Age at Maturity of Icelandic West Coast Stocks of Atlantic Salmon (Salmo salar)

Investigations were conducted on the effects of oceanic variations (as measured by sea temperatures) and catches by the West Greenland salmon fishery on the sea age composition of Atlantic salmon (Salmo salar) stocks from 21 Icelandic west coast rivers. Annual ratios of grilse to two-sea-winter (2SW) salmon were strongly correlated among the 21 rivers. All eight rivers with time series extending back before the expansion of the West Greenland fishery showed lower ratios during the earlier period. Only 2 of the 21 rivers, however, had significantly declining ratios over their time series. In addition, for only one river was West Greenland catch significantly related to the ratios (P < 0.05), and for only one river did ratios increase when the expanded West Greenland fishery was active. Overall, the effects of the fishery on stock composition are evidently minimal. The mean April–May temperature when the smolts were to migrate out of rivers was significantly and positively related to subsequent ratios for five of the rivers, which, along with correlations among the ratios, indicated that more rapid growth of smolts in their first summer may have increased the ratios of grilse to 2SW salmon on several rivers.

THE POTENTIAL ROLE OF LEGUMES IN THE REHABILITATION OF LEVELLED GOLD DREDGE TAILINGS, TARAMAKAU RIVER, N.Z.

Recent increases in the value of gold have led to renewed interest in gold mining - particularly by the dredging of the river flats bordering West Coast rivers. In any future licensing of such dredging, the rehabilitation of the "destroyed" land will be a major consideration. Nitrogen is the main nutrient limiting the growth of radiata pine on the gold dredge tailings; this paper outlines initial results of an exploratory legume-fertilizer trial on levelled tailings in the Taramakau River valley. Twelve legume species were grown with and without superphosphate (+ MO, Mg and K). Tree lucerne (Cytisus proliferus) , Russell lupin (Lupinus polyphyllus) and yellow tree lupin (L. arboreus) grew well without the application of fertilizer but the pasture legumes had substantially increased dry matter production where P was added - in white clover (Trifolium repens) dry matter production was increased from 5.4 to 8.9 t/ha; and total N was 180 and 310 kg N/ha. Tree nutrition and growth were greatly improved by association with a vigorous legume species, In addition, the presence of legumes improved the appearance of the tailings and with a pasture species grazing potential was created