Connectivity between the marine coast and estuary for white mullet (Mugil curema) in northeastern Brazil revealed by otolith Sr:Ca ratio

Abstract

Microchemical analyses were carried out in order to estimate the Strontium:Calcium (Sr:Ca) ratio in the otolith of the white mullet, Mugil curema, in the Pernambuco (at the Santa Cruz channel, Brazil) in order to determine its connectivity between the estuary and ocean. Variation in the otolith Sr:Ca ratio was directly related to salinity, with greater salinity denoting a higher otolith Sr:Ca value. Data on the otolith Sr:Ca ratio demonstrates that the individuals analyzed are born in areas of salinity that are characteristic of the estuary, where they develop until approximately one year of age, at which point they migrate to areas of greater salinity until reaching sexual maturity (3 years of age) in the sea. Spawning occurs in the ocean, after which M. curema individuals may either remain or return to the estuary until the next spawning. Differences in estuarine salinity were found for young-of-year individuals and may be related to the season when spawning took place, since M. curema females are found spawning throughout the year. The hypothesis is that higher salinity in the dry season leads to a greater otolith Sr:Ca signature among individuals spawned in this season from birth until one year of life. On the other hand, the lower salinity in the rainy season leads to a lower otolith Sr:Ca signature among individuals spawned in this season. These information are important for the adequate management of the white mullet stock in northeastern Brazil.

Introduction

Fish from the family Mugilidae are distributed throughout the tropical and subtropical regions of the world, mainly in estuarine and coastal waters, where they are commercially exploited by artisanal fisheries, displaying economic and social importance to human populations (Menezes et al., 2015). According to FAO statistics, Brazil accounted for an annual mean of 10.9% in weight of the global production of mullets between 1950 and 2004, ranking the eighth largest producer in the world (FIGIS-FAO, 2006).

Seven species of mullet are found in Brazil – all from the genus Mugil (Menezes et al., 2015). Of these, three are found in Pernambuco state (northeastern Brazil), among which M. curema is the most abundant accounting for 96% of mullet catches (Santana da Silva, 2007). M. curema has euryhaline habits and can be found throughout Brazil's coast. The species uses estuarine regions as nurseries, while spawning occurs at sea (Ibáñez Aguirre, 1993; Ditty and Shaw, 1996; Marin et al., 2003; Ibáñez and Gutiérrez Benítez, 2004; Ibáñez et al., 2012).

A number of studies confirm the correlation between the otolith Sr:Ca ratio and salinity (Secor et al., 2001; Zimmerman, 2005; Labonne et al., 2009), whereas others have not found this correlation (Elsdon and Gillanders, 2002; Rooker et al., 2004). Thus, other factors likely interact with salinity, such as the water chemical composition, temperature or ontogenetic and/or physiological changes (Kawakami et al., 1998; Elsdon and Gillanders, 2002, 2003, 2004, 2005; Pontual et al., 2003; Martin et al., 2004). As the incorporation of elements and the salinity effects on otoliths differs from species to species, thereby limiting the determination of general validation models, studies for specific validation are needed to confirm the relation between salinity and the Sr:Ca ratio in otoliths (Secor et al., 1995; Tzeng, 1996).

In tropical regions, there is little variation in temperature (2–4 °C) during the year (Santos et al., 2000). However there is seasonality regarding rainfall, which leads to differences in salinity between rainy and dry seasons. In the Santa Cruz channel, the largest estuary in Pernambuco, salinity ranges from 7 to 10 in the rainy season (March to August) and from 34 to 37 in the dry season (September to February) due to the rainfall patterns (Medeiros and Kjerfve, 1993). In this region, M. curema is under fishing pressure, especially in the estuary, where there is a high frequency of juveniles (Santana da Silva, 2007). Hence, studies investigating the use of estuarine and marine environments by this species are necessary to better manage its stock.

To date, only one study was conducted on M. curema's migratory pattern using otolith Sr:Ca ratios (Ibáñez et al., 2012), and none in Brazil so far. The present study aimed at identifying the migratory pattern, connection, and environmental history between the estuary and ocean for M. curema. A comparison of the transects of Sr:Ca ratio from the nucleus to the edge of the otolith enables an assessment of which environments should be protected in order to assist in the species management. Variations in the otolith Sr:Ca ratio and salinity are also studied in order to validate the relation between these variables for the species.