Environmental and ecological indicators of fish mass-mortality phenomena in mediterranean reservoirs.

Abstract

Reservoirs are the preferred locations for anglers in Mediterranean regions. However, such ecosystems are impaired by a set of characteristics and phenomena that can constrain the suitable management and exploitation of related goods and services, especially in the current climate change context. In these reservoirs, eutrophication is one of the main water quality problem. These systems are often affected by large nutrient inputs and accentuated water-level fluctuations, especially during summer, that lead to a decrease in habitat availability and quality. Fish density tends to increase which, together with a low oxygen concentration, contributes for massive fish mortalities, compromising a valuable resource for recreational fisheries. Knowledge about the environmental and ecological factors that are directly related with fish biomass increases in these ecosystems, is extremely important to promptly identify this environmental risk and minimize its occurrence. This study, developed for the project “GAMEFISH - Management of Mediterranean reservoirs for the promotion of recreational fishing activities”, aims to identify the main environmental and ecological indicators of fish mass-mortality phenomena in southern Portuguese reservoirs, by evaluating: i) which are the fish species, and respective biomass thresholds, that contribute more to this phenomenon; and ii) which environmental variables are related with population changes and can be used to predict the probability of occurrence of fish mass mortalities. During one-year, monthly fish sampling campaigns were conducted in five reservoirs from the south of Portugal (Póvoa e Meadas, Penedrão, Serpa, Tourega and Divor), using a mix array of electrofishing and multi-mesh gill nets. Several environmental variables were collected for each reservoir-sampling campaign combination, including average depth, Secchi-Disk, water temperature and dissolved oxygen (both continuously registered at 15-min intervals) and water samples for the analysis of quality parameters (e.g. chlorophyll-a, CBO5, phosphorus, nitrates, bacteria). Complementarily to the in situ characterization, each reservoir was also monitored through Sentinel-2 (S2) satellite image products, which provide high-resolution optical imagery for land services. Obtained results reveal significant differences between the monitored reservoirs concerning the temporal variability of their fish assemblage composition, abundance and biomass, as well as their abiotic characteristics, particularly dissolved oxygen, which in some cases (i.e. Tourega and Divor) drastically diminished into values below the limit for fish survival (<3 mg/L) during summer. In some of the studied reservoirs, significant biomass increases and subsequent fish mass-mortality were detected (August-October 2017), which caused accentuated changes in the abundance and biomass of some species, less tolerant to environmental degradation, such as the non-indigenous largemouth bass and bleak, and native Iberian nase. During the study, two of the monitored reservoirs (i.e. Penedrão and Divor) were subjected to a national plan of fish removal, with distinct impacts in the respective communities, which also allowed to evaluate the effectiveness of such management effort. Outcomes from this study will provide owners and managers of these reservoirs with monitoring information capable of promoting a timely prediction and avoidance of massive fish mortality phenomena.