Determining seasonal dominant fish eggs and larvae species groups in the west of Tonkin Gulf

From 2003 to 2016, 1,649 samples were collected, covering both the time and space of the Gulf of Tonkin. The results have identified groups of seasonal dominant fish eggs and larvae: Seven species groups in Spring, nine species groups in the Summer, six species groups in Autumn, and four dominant species groups in the Winter. The dominant index (Yi) ranges from 0.02 to 0.26 depending on the species group and each season of the year, the highest in the Goby group - Gobiidae (Yi = 0.26) achieved in the Spring, followed by Herringgroup - Clupeidae reaches Yi = 0.20 in the Summer and the Anchovy group - Engraulidae reaches Yi = 0.16 in the Summer. The highest advantage index is only Yi = 0.09 in the Winter for Unicorn cod species - Bregmaceros macclelandi. During this period, the number of taxa and dominant indexes tended to decrease from Spring to Winter slightly.

Effect of freshwater discharge from Namgang Dam on ichthyoplankton assemblage structure in Jinju Bay, Korea

The movement of fish eggs and larvae in bay and estuarine systems is affected by freshwater discharge. In this study, the assemblage structures of ichthyoplankton eggs and larvae were assessed for the first time in Jinju Bay, South Korea, to identify the spawning and nursery functions of the bay. Fish eggs and larvae and several environmental parameters were sampled monthly from April 2015 to March 2016 inside and outside of the bay. Within the bay we collected eggs and larvae from 25 and 35 species, respectively, indicating greater diversity than outside the bay, where we collected eggs and larvae of 20 and 28 species, respectively. Fluctuations in water temperature and salinity were larger inside than outside of the bay, and chlorophyll-a concentration was higher within the bay, likely due to discharge from the Namgang Dam, which causes water to flow from the inside to the outside of the bay. This process influences fish larva abundance, such that more larvae are found outside than inside the bay. We also found that 28 fish species use Jinju Bay as a spawning ground. For some species, the timing of egg and larva appearance differed inside and outside of the bay, suggesting that the timing of spawning may differ between the two environments.

Drifting in a free-flowing river: Distribution of fish eggs and larvae in a small tributary of a Neotropical reservoir

We investigated spatial and temporal variations in the distribution of fish eggs and larvae in the Guareí River, a free-flowing tributary located in the upper section of the Jurumirim Reservoir, Paranapanema River, Brazil. Fish eggs and larvae were sampled ~ fortnightly from November 2017 to March 2018 (Cycle 1), and from November 2018 to March 2019 (Cycle 2), at three sites distributed along the entire course of the Guareí River. We collected 859 fish eggs and 1,340 larvae, comprising 19 taxa. We found reproductive activity of several fish species in the Guareí River, including long-distance migrants. However, there was no spatial differences in the assemblage structure and densities of fish eggs and larvae along the channel of Guareí River. We recorded temporal differences in larvae density only in Cycle 2. The consistent capture of fish eggs and larvae in two reproductive cycles is strong evidence that this river is a spawning site for a variety of fish species. Thus, we recommend maintaining its natural flow regime.

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

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.