Metapopulation model for yellow snapper, Lutjanus argentiventris, and its implications for fisheries and conservation in the Gulf of California

Researcher:  Octavio Aburto (maburto@ucsd.edu) 

Overview
 The yellow snapper Lutjanus argentiventris is the most abundant snapper and most commonly fished species in the Gulf of California. This species reaches sizes of up to 1 meter in length and a weight of 25 pounds. It is a gonochoristic species, which means that each individual is either a female or a male; however there is no sexual dimorphism. Tthe life cycle of the yellow snapper, exemplifies both essential habitats that maintain this cycle (mangroves and deep rocky reefs). The connection between these habitats is through the larvae, which reach the mangroves after their planktonic stage, and through the migration of sub-adults that reach the deep seamounts once they leave the mangroves and spend time in shallow rocky habitats.

This project will provide the first data about the survivorship of the individuals of a commercial species after their nursery time in mangroves. In particular with snapper species, this proposal will be the first study that will integrate the different rates of  mortalities on every life stage from the larvae that reach the mangroves to the migratory individuals that leave them.

Results
Larvae spend an average of 21 days in the water column before they settle. The most important settlement peak in mangroves is from September through November. These settlement patterns are not related with any moon phase.

There is an ontogenetic shift in microhabitat requirements of yellow snapper juveniles. After the larvae reach the mangrove s cove, settlers look for refuge in the crevices between pebbles. These individuals are between 1 and 2 cm in length, and they have an extremely narrow home range.  Most of the time, pebbles are located in the mouth of the coves and, because they remain submerged during low tide, settlers do not need to move during the day. After they reach between 4 or 6 cm in length and spend around 15 days among the pebbles, they move to mangrove roots to find shelter and food. Here they will spend the rest of their nursery time, which can take between 6 and 8 months. 

Daily growth of juvenile yellow snapper is around half centimeter ( 0.41 mm/d) consequently  they can reach 15 centimeters in length during their first year. Once they leave the mangroves the growth rate decreases exponentially. Changes in growth rate are related to the diet in the different live stages. Settlers diet consists mainly of hermit crabs and amphipods. When they move to mangrove roots their diet changes to bigger invertebrates such as blue crabs, mangrove crabs, land crabs, oysters and mojarra fish. During some seasons food in mangroves is subsidized from open marine systems like squid that come to die in the shore or schools of tiny herring that come to find refuge from predators. Once snappers leave the mangroves they become active night predators and their diet includes animals from rocky and sandy bottoms such as lobsters, shrimps, mantis shrimps, snake eels, and scorpion fishes. They also prey upon animals from the water column especially mysid shrimps, pilchards, and silverside fish. An intensive settlement peak occurs during the summer season. The availability of pebbles in Balandra is not large compared to other mangroves in Baja, so we recorded settlers less than 3 cm inside the mangrove roots. Settlers can be seen until late winter, but the pulses are weak compared to the summer ones. Contrary to our expectations, annual cohorts are well defined and can be followed throughout time, until the individuals leave the mangrove and use the boulders outside the mangrove cove. We also found that they tend to avoid the sandy beaches in the right side of the bay  99% of the individuals were counted in the left side of the bay.

Final Remarks
We have described the processes that take place during the nursery stage for yellow snapper. Important findings are: an ontogenetic shift in the requirements of microhabitats, a 6-8 months nursery time for juvenile individuals, and one intensive settlement peak in summer; however, settlers can be seen until late winter. Mangroves provide an intermediate nursery habitat that may increase the survivorship of young fish.
Growth rates are similar to other snapper species. Ontogenetic changes in diets are extremely close with the changes in growth rates. The yellow snapper is a night generalist predator, with preys present on the reefs and on the sandy boundaries.
Our research is not concluded, and we are analyzing the data with statistic matrixes in order to establish the mortality rates for the different size classes. However the magnitude of snapper exportation rates is several thousands of snapper per annual cohorts.
 

Funding from:

PADI Foundation