Net cages enhance golden mussel (Limnoperna fortunei) larval density and condition factor

Abstract

Cage fish farming is an important economic activity in Brazil where tilapia (Oreochromis niloticus) is the main cultivated species. To investigate the cage fish farm as a vector for golden mussel invasion, we hypothesised that: (1) Limnoperna fortunei, a freshwater bivalve mollusc, finds greater food availability in the cage fish farm; (2) the cage fish farm favours mollusc colonisation and its nutritional status; and (3) its performance in the reservoir would vary over the year. We obtained data from limnological variables as well as the number of larvae and adult individuals, to evaluate condition index (CI) monthly from February 2015 to January of 2016. We established sampling stations at three points with fish farming and three points without fish farming (controls) in the Canoas II Reservoir, Paranapanema River, Brazil. We used principal component analysis (PCA) to identify spatial and temporal distribution of samples according to limnological variables. We performed a linear mixed model analysis to test the hypothesis of no difference between areas (fish farm and control) and time (months) for larval density and CI of L. fortunei. The first PCA axis (28.48% of variance) revealed a temporal gradient, showing the separation between cold and hot months. The second PCA axis (19.61% of variance) separated the samples of control and fish farms; fish farms displayed higher concentrations of total phosphorus, nitrite, and chlorophyll a. Larval densities of L. fortunei were higher in fish cage areas with time being responsible for 35% of this variation. Mussel CI was also higher in fish farms and time was responsible for 36% of the variation. We conclude that fish production in cages contributes to the establishment of golden mussels in the reservoir. Our results serve as a warning that other molluscs or invasive species may be favoured by aquaculture activities in aquatic environments. The colonisation of the mussel in the reservoir varies during the year in response to seasonality.