Human-induced influence on eggs and larval fish transport in a subtropical estuary

Abstract. The transport during the early stages of life to the nursery areas is one of the main processes in the maintenance of the marine fish population, and human interventions can interfere with this process. In this work, the TELEMAC-3D model coupled to passive particles was used to understand the effect of the change in the configuration of the Barra Jetties of the Rio Grande regarding the transport of eggs and larvae of the croaker Micropogonias furnieri in the Patos Lagoon estuary (PLE). Twelve experiments of 5 days that consisted of periods of high and low discharge combined with winds from the south quadrant (SW, S, and SE) were carried out to test the hypothesis that human interventions in the coastal region alter the transport patterns of fish eggs and larvae. The low flow guaranteed a greater extent of saline intrusion and larvae incursion in the estuary, with the opposite occurring in the scenario of high flow. The SW wind ensured the most efficient recruitment into the estuary, in terms of both entry time and maximum reach in both configurations. However, the recent modernization works of the Barra Jetties have changed the pattern of transport and dispersal of larvae and have reduced the amount and reach of the incursion of croaker eggs and larvae into the estuary compared to their old configuration. With the new configuration of jetties, reductions in the larvae concentration and abundance in the estuary were registered at approximately 25 % for SW and S winds, 68.6 % for SE wind at high discharge, and 0.5 % to 1 % for winds at low discharge. The lateral stratification in the access channel to the estuary, an important parameter in the larvae transport and distribution between the jetties and the predominant wind direction, was decisive in defining the initiation time of the stratification. With the old configuration, the lateral stratification was established 1 h, 7 h, and 10 h after starting the simulation with the incidence of SW, S and SE winds, respectively. In the new configuration, the lateral stratification was established at the same time only with the SW wind, but with a reduced salinity gradient. In this configuration, only the beginning of stratification was observed at the estuary mouth with S winds, while the stratification was not established with SE winds. This fact influenced the intrusion of saline water and resulted in a smaller number of larvae between the jetties and consequently their transport into the estuary. With the new configuration, a reduction in the maximum penetration of the larvae within the estuary was observed at 1.6 km for high discharge and 2.3 km for low discharge. Despite limitations inherent to the numerical modeling technique, the results obtained corroborate the hypothesis that human interventions in the coastal region change the patterns of transport of fish eggs and larvae. Furthermore, the findings suggest that modernization works of the jetties have contributed to reducing the transport of dependent estuarine species to the Patos Lagoon estuary. Coupled with the knowledge obtained by other research about this species, this knowledge can support provisioning measures for better management of fishery resources in the region.