Fishery ecology of the freshwater fishes in the Lake Nicaragua. Reproduction and management of Brycon guatemalensis

Abstract

The freshwater bodies are very dynamic systems, and have been the first aquatic ecosystems susceptible to receive the direct impact of human activities, becoming very vulnerable ecosystems. The Lake Nicaragua has been recognised as a continental water body with an important source of fish living resources for the fishery sector. In addition, its dimension is very attractive for the development of aquaculture activities. However, the exploitation level of fishing resources in the lake has lead to an overexploitation of some species already included in UICN red list, while others are in risk of being overexploited if management measures are not taken. Studies on species diversity in the lake defined two demersal fish habitats: the ¿shallow benthic¿ habitat, which has a higher index of species diversity and the ¿deep benthic¿ habitat with lower diversity index. The landings data series do not allow ascertaining the catches composition. Hence, any evaluation of the fish exploitation pattern in the Lake Nicaragua derived from landings record is imprecise. Besides, the large gap on knowledge of many other important biological parameters of the fish species - the dynamic of reproductive behaviour of the population, the reproductive cycle, spawning ground, egg production and the maternal features affecting reproductive output and fitness- prevents the implementation of management actions directed towards the conservation of the lake stock reproductive potential, such as regulations on closing season and/or areas, and minimum landing sizes. This study, besides providing information about actual state of distribution and abundance of Amphilophus citrinellus, Hypsophrys nicaraguensis, Parachromis managuensis, and Brycon guatemalensis, provides information of the reproductive strategy of B. guatemalensis and proposes some management alternatives based on its biological features. The present study was conducted along the eastern part of the Lake Nicaragua and showed that even all the species studied were widely distributed in this area, species abundance varied geographically (zones: northwest, central and southeast) and vertically (depth: from shallow to deeper areas). The patterns of abundance of cichlid (A. citrinellus, H. nicaraguensis and P. managuensis) are affected by spatial (zones) and environmental factors as the vertical profiles (depth), whereas in B. guatemalensis was also affected by seasons of the year, i.e., dry and rainy season. Cichlids were more abundant in shallow waters and in the southeast zone, whereas B. guatemalensis larger abundances were found in the southeast and northwest side. Similarly, the size distribution pattern was influenced by both latitude and vertical profiles. Overall, larger fishes are found in the southeast and to a lesser extent in the central zone of the lake, whereas in the northwest are smaller. Furthermore, larger fish are found in relative deeper waters. The reproductive studies on B. guatemalensis using histological procedures showed that the oocytes final maturation ended with the migration of the germinal vesicle to the animal pole, without oocyte hydration, and oocytes being surrounded by mucus indicative of eggs adhesiveness. The species presented a protracted spawning season that lasted 8 months. Both small and large females started the spawning activity in July, but spawning asynchrony occurred between female sizes as spawning season progressed. Based on histological procedures, the female length at 50% maturity of B. guatemalensis was estimated in 27.3 cm, which largely differed from the ones previously reported based on the gonad macroscopic observations. The present study indicated that B. guatemalensis has determinate fecundity and presented group-synchronous ovarian organization. The potential annual fecundity of the species showed that this allometrically increased with female size, and that larger females produced larger eggs. Nonetheless, egg size decreased as spawning season progressed. Gillnet selectivity studies were performed on four species (A. citrinellus, H. nicaraguensis, P. managuensis, and B. guatemalensis) and selectivity was quite different among species, partly reflecting growth patterns. The absence of larger and smaller fishes was notorious in the catches. The absence of larger fish likely indicated the expected maximum size of each species within the lake ecosystem, while the minimum size captured may be attributed to the fish shape and/or different behaviour of the life stages of each species. The most vulnerable fraction of the population of cichlids was males, and females in the case of B. guatemalensis, particularly during reproductive periods. The sex ratio of each species obtained in the present study very likely is similar to those in the catches in a routine fishing operations performed by the fishers in the lake. The establishment of a minimum landing size (MLS) is one of the most common management measures, particularly in data limited stocks exploited by the small-scale fisheries, and often this measure is reinforced with regulations on fishing gears mesh size. Generally, legal mesh size is determined combining gear selectivity and length-at-50% maturity of the target species. The main objective of this management rule is the protection of the immature or juvenile components of the stock, a measure often considered enough to ensure the sustainability of the fishery. However, reproductive studies have evinced that larger mature females are more productive than smaller mature ones, thus, the catches of larger fish could also lead to a significant reduction of stock productivity. In this study, we analyzed the impact of gillnet selectivity on B. guatemalensis in relation to ontogenic maturation, reproductive phase, and reproductive potential. Results showed that juveniles and recruit spawners are the most vulnerable to the 75 mm mesh-size, whereas the majority of the spawning stock biomass (SSB), and particularly those with higher reproductive potential, are highly vulnerable to the 100 mm mesh size, due to the interaction between the female length-mesh relationship and the seasonal behaviour pattern of those females. Based on these results, a MLS of 30 cm is proposed as management measures to protect the immature or juvenile components of the stock, and to increase egg production of the SSB and reduce the fishing mortality of young females we recommended the use of 100 mm mesh size of net for fishing operation in the Lake Nicaragua.