Refúgios para peixes na seca em um lago de várzea: Implicações para o manejo pesqueiro na Amazônia Central

Abstract

In the low water season, the retraction of the aquatic environment increases the density of fish, making them more vulnerable to fishing. In addition, an increase in biotic interactions such as competition and predation, and intensification of the effects of abiotic factors that may reach critical limits, such as dissolved oxygen (DO), temperature and total solid suspension (TSS), is expected. Given this, some fish seek strategies to survive these periods, either with physiological adaptations or seeking refuge. Lake Janauacá, located in Manaquiri-AM, is a mixed lake of floodplain and ria, the latter surrounded by an area that is not annually flooded, called terra firme. Ria lakes are formed by flooding of river valleys, are dendritic in shape and tend to be deeper and steeper than floodplain lakes (marginal or island lakes). This work aimed to identify in Janauacá Lake, places of refuge for fish in the low water season, which may be important for fisheries management strategies. For this, the lake was divided between northern region (floodplain lake) and south (ria lake). Four data collection events were conducted, two in falling water and two in low water, eight fisheries per collection, totaling 32 fisheries (16 in the north and 16 in the south). The fish were collected with a mesh battery, composed of four nylon waiting nets with different monofilaments, totaling 20 m in length and 2 m in height. The nets were immersed for 24 hours with fishing every six hours. While the nets were armed, environmental conditions such as temperature, DO, TSS and depth were characterized. At the end of each fishery, the fish were identified and photos were taken to confirm field identification. From this, the Capture Per Effort Unit (CPUE) (number of fish / total mesh area) was estimated. CPUE and fish diversity were analyzed for environmental variables measured in both regions and by hydrological periods. The northern lake region, in the low water season, presented high TSS, temperature and low DO levels, so, according to the fish found in this region, the northern lake can be a refuge for species that seek protection from predators and are adapted to lower concentrations of DO. The southern part of the lake, in the same hydrological period, presented higher levels of DO, depth and lower of TSS and temperature, suitable conditions for hypoxia intolerant species and visual predators. CPUE was positively related to DO and was higher in the south of the lake. The diversity was different in the two collection regions, but did not present significant relationship with the environmental variables tested. Being larger in places with flooded vegetation, it was higher in the northern part during the falling water, but during the low water the regions did not present diversity among themselves, since the available environment is open water, without vegetation. Since both regions of the lake have been shown to be havens for commercially important fish species, it is interesting that mixed floodplain and ria lakes provide protection targets, especially in low water, where conditions are more restrictive and fish are more vulnerable to fishing.