Reproductive Biology of the Striped Red Mullet, Mullus Surmuletus, and Implications for Its Management

Reproductive biology of the striped red mullet, Mullus surmuletus, and implications for its management

Joaquim Tomàs-Ferrer*1, Ulla Fernandez-Arcaya2, AmàliaGrau3, Beatriz Guijarro2, Natalia González2 and Enric Massut´ı2

*Corresponding author: [email protected] 1Universitat de les Illes Balears 2Instituto Españolde Oceanograf´ıa- Centre Oceanogràficde Balears 3Laboratori d’Investigacions Marines i Aqüicultura

Keywords: Fecundity, Maturity, Mullus surmuletus, Reproduction

Introduction

Mullus surmuletus Linné,1758 is one of the most important target species of Iberian Peninsula fisheries, including the Balearic Islands, where the official registers of annual catch vary between 100 and reaching maximums of 400 tones. However, in the last 7 years, annual catches have been decreasing and now they oscillate between 60 and 100 tonnes. Despite its economic relevance, some of its reproductive aspects are still unknown. The objective of the present study is to characterise the reproductive ecology of M. surmuletus including the reproductive cycle and strategy, fecundity, population distribution and size-at-maturity of this species. This study was based in the analysis of 1998 individuals captured with two different fishing gears, trammel and trawling, between 13 and 518 m depth (mostly at depths inferior to 100 m), covering an entire year of sampling. The populational structure of this species showed a marked bathymetric pattern, with the presence of larger individuals at shallower waters (< 50 m depth), as well as a marked sex ratio segregation between shallow and deeper waters. Macroscopic and microscopic analyses of the gonads as well as Gonado Somatic Index showed a seasonal reproductive cycle with the presence of spawning females from March to May, while males spawn from January to May. The size-at-maturity was 14.4 cm for females and 12.2 cm for males. The size-frequency distribution of oocyte diameter showed an asynchronic ovarian organisation. The counting of mature oocytes in females that were just about to spawn showed a batch fecundity of, approximately, 74,500 ± 40,000 oocytes per female. Mullus surmuletus is an Actinopterygii fish, from the family Mullidae. It is distributed in areas of the Atlantic Ocean and the Mediterranean Sea. The red striped mullet is a demersal species with a bathymetrical range of 10 to 400 m depth, living on rocky or sandy bottoms or seagrass meadows (Reñoneset al., 1995). Mullus surmuletus has a high commercial value and is an important resource for the Mediterranean demersal fisheries. This species is captured by two different fishing gears in the Balearic Islands: trawling and small-scale fisheries. Mullus surmuletus biology has been poorly studied. Despite its relevance, there is still an important gap related to the reproductive biology of the red striped mullet, especially in the Mediterranean Sea. The lack of information about the fecundity and reproductive strategy of this species, including the fact that there are no recent studies on relevant biological aspects as size-at-maturity, represents a great opportunity to make a current approach of those parameters to verify if they have varied during the last years, comparing them to older ones. The lack of currently gathered biological data on reproduction of M. surmuletus is an obstacle to conduct a sustainable stock assessment of this resource. The main goal of this study is to describe the reproductive biology of M. surmuletus in the Balearic Islands and to provide new and updated scientific knowledge that can improve the management options for this species in the Balearic Islands.

1 Material and methods

A total of 1998 specimens of M. surmuletus were collected monthly for each fishery gear during the period from October 2017 to September 2018. A biological sampling was done, measuring and weighing individuals, then weighing its gonads and hep- atopancreas. Sex was also defined. Gonads were examined macroscopically in order to determine its matu- rational stage. Gonads were fixed in formalin for histological sampling. 195 gonads were sampled in order to calculate the fecundity of this species by gravimetric method and other analysis, consisting on determine the microscopic gonadal maturation stage and measuring oocyte diameters. Data analysis was done in order to find significative differences in size-frequency distributions of captures (Kolmogorov-Smirnov tests) and in sex ratio (Chi-square tests).

Results and discussion

Individuals collected ranged between 10 and 34 cm. Small-scale captures showed a unimodal-like length- frequency distribution during all the seasons. On the other side, trawling captures showed a bimodal-like length-frequency distribution, although it was not so evident in summer. Recruitment of juveniles to adult stock has been observed to occur in November. Our results showed a marked bathymetric segregation with the presence of larger individuals as well as higher proportion of females at shallower depth where trammel fisheries (< 50 m) occur. Sex ratio of this species was 2.2 females for every male. Individuals captured in shallower waters are mainly females (sex ratio 6.1:1) whereas those captured in deeper waters (by trawl) present a more balanced sex ratio (1.6:1). Sex ratio varies for each length class of M. surmuletus. As the size increases, females become more predominant. This huge skew could be provoked by the fact that only a few males (in addition, the largest) were found in shallow waters, clearly predominated by females. Spawning capable females were observed from February to May, with maximums in March (24%) and April (25%), and actively spawning females from March to June, prevailing in March (72%), April (61%) and May (73%). Regressing females were observed from March to July, being most common in June (40%). The spawning pattern identified histologically in females and males seems to be corroborated by the seasonal variability in the GSI. HSI values follow similar pattern than GSI. The reproductive activity of M. surmuletus seems to occur during the end of winter and extending to the late spring (Pajuelo et al., 1997). Our results showed that the individuals captured by trawling, at deeper depths, showed higher of GSI than individuals captured by trammel at shallower waters, suggesting that this species might migrate to deeper ranges for spawning, although larger females and males are found in shallow waters. Despite this bias in size, larger females captured by trammel presented similar ratios of gonads development phases and oocyte development stages. An explanation for the resulting lower GSI values is that the gonads weight – female weight relation may be allometric. Size-frequency distribution of oocyte diameter showed the presence of oocytes from all the developmental stages during the spawning seasons on ovaries of actively spawning females without observing a bimodal ovarian organisation. These results suggest that this species present an asynchronous ovarian organisation (Murua et al., 2003). The different stages are a consequence of a continuous development process, any evident hiatus is observed between the immature and vitellogenic oocytes nor between vitellogenic and germinal vesicles migration stages oocytes. The estimated mean total length (L50) at which 50% of females attain maturity was 14.36 cm. The L50 for males was 12.18 cm, which represents a decrease of 2.4 cm for females and 2.8 for males from previous published data of this species on the same study area. Above 17 cm, all individuals were mature. In females with hydrated oocytes, the number of developing oocytes (NDO), considered as those in vitellogenic stage or superior was of 334,000 ± 76,000 oocytes. Batch fecundity, which is the number of migrating and hydrated oocytes, was 74,500 ± 40,000 oocytes. Relative number of developing oocytes (RNDO), which is the NDO per gram of female, was estimated to be 2,200 ± 370 and the relative batch fecundity was 450 ± 200 oocytes per gram of female for total and batch fecundities, respectively. NDO and batch fecundity increase potentially with the female body weight.

2 This study provides new key data on aspects of reproductive biology of M. surmuletus in the Balearic Sea, where it constitutes a specific stock unit. The results of this work will contribute to the knowledge of the reproductive biology of M. surmuletus in the Balearic Sea and provide actualised information on several of its life parameters. This study corroborates previous studies’ calculi of the sex ratio, confirming a female-male relation close to 2.2:1 (Morales-Nin, 1991). The results on this study show a bathymetric migration behaviour on this species. Seasonal variations in the size-frequency distribution are mainly due to the life cycle of the species. Smaller individuals are captured in autumn, when recruits join the adult population. This study also confirms that this species has a well delimitated spawning season that lasts from March to May, with a peak that oscillates between late April and early May. This species has confirmed to be an income breeder rather than a capital breeder, since it does not storage energy resources before reproduction to be used but it uses energy directly from food intake. Fecundity was calculated for first time in this species in the Western Mediterranean with results quite similar to the species M. barbatus, although it has slightly smaller sizes. More studies with larger sample sizes will be needed to be done in order to confirm or refute the results obtained. However, this data obtained is now available for the species’ fishery management and the construction of stock assessment models. Results obtained in this study, together with the characteristics of the landing patterns observed in M. surmuletus, reveal the need of carry out a continuous scientific monitoring on this species. These results suggest that several parameters of reproductive biology of M. surmuletus are quite similar to the ones of the other Mediterranean species of Mullus, M. barbatus (Ferrer-Maza et al., 2015). In the Mediterranean, a minimum landing size of 11 cm has been stipulated by the European Commission in 1994. This value is still lower than all the L50 calculated in this study since 2001 and lower than the majority of L50 provided by many authors for Mediterranean populations. We thereby suggest an increase of the minimum landing size closer to the L50 calculated to this species, in order to ensure the M. surmuletus captures under the maximum sustainable yield, since the species is catalogued as overexploited with relative high biomass by FAO.

Acknowledgments

Data and samples used in the current study have been provided within the framework of the Programa Na- cional de Datos Básicosdel Sector Pesquero, reguled by the Regulation (EU) 2017/1004 of the Parliament and Council, at May the 17th of 2017, which stablishes a communitarian framework (Data Collection Framework) for the recopillation, management and use of data from the fisheries sector, and assistance to the scientific assessment, in relation to the Common Fisheries Policy (CFP) of the European Union, and co-financed by the IEO and the EU thorugh the European Maritime and Fisheries Fund (EMFF).

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