Abstract
Fish assemblage was examined for patterns in spatial and seasonal structure within an estuarine coastal lagoon, Ria de Aveiro. Two years of variation in abiotic conditions were recorded to identify factors responsible for maintaining the structure of fish assemblages. Nine sites were sampled monthly with a traditional “chincha” beach-seine net between November 1998 and October 2000. Fish abundance and biomass changed significantly between sites. Temperature was found to be the most important abiotic factor affecting the abundance (number of fish) distribution of fish assemblage, which is more related to seasonal migrations than to spatial migrations. Salinity was the most important abiotic factor affecting the fish biomass, with temperature also having a major influence. The other abiotic parameters analysed also showed some influence on the distribution of fish, although, isolated from the other controlled and/or uncontrolled factors, they could not fully explain all the differences between assemblages. Feeding analysis indicates that fish assemblage is mostly dependent on small benthic and pelagic crustaceans and/or detritus, although they can feed opportunistically on other prey. The low spatial segregation of fish in relation to feeding preference indicates that, in a food-rich lagoon such as Ria de Aveiro, the environmental-biological interactions appear to have a greater effect on fish distribution than do biological interactions.
Highlights
Assemblages of estuarine organisms vary in time and space, largely because estuaries have widely varying environmental characteristics and serve as nurseries for many productive and dependent marine species (Rogers et al, 1984 in Rakocinski et al, 1996; Elliott and Hemingway, 2002)
The habitats and fish assemblages are potentially affected by several anthropogenic influences which may have a direct influence on the food resources, distribution, abundance, growth, survival and behaviour of the fauna present (Whitfield, 1996)
The assessment of fish assemblage structure has a high value in estuarine quality assessment (Elliott et al, 1988; Pomfret et al, 1991) causal links have to be determined between environmental parameters and fish assemblage structure and functioning
Summary
Assemblages of estuarine organisms vary in time and space, largely because estuaries have widely varying environmental characteristics and serve as nurseries for many productive and dependent marine species (Rogers et al, 1984 in Rakocinski et al, 1996; Elliott and Hemingway, 2002). The main estuarine water quality parameters, salinity, temperature, turbidity, pH and dissolved oxygen are known to affect fish distribution (Blaber and Blaber, 1980 in Marshall and Elliott, 1998). Substratum heterogeneity and vegetal cover affect fish distribution through their influence on prey availability (Marchand, 1993) and/or protection from predators (Blaber and Blaber, 1980 in Marshall and Elliott, 1998). The assessment of fish assemblage structure has a high value in estuarine quality assessment (Elliott et al, 1988; Pomfret et al, 1991) causal links have to be determined between environmental parameters and fish assemblage structure and functioning. Identification of significant associations between fish species and habitat conditions is the first step towards incorporating environmental information into fish abundance (Perry et al, 1994)
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