Spatio-Temporal Distribution and Population Structure of Parapenaeus Longirostris and Nephrops Norvegicus in the Northern Alboran Sea

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Spatio-temporal distribution and population structure of Parapenaeus longirostris and Nephrops norvegicus in the northern Alboran Sea

Cristina Ciércoles*1, Pere Abelló2, Pedro Torres3, JoséMiguel Serna3, Jose Luis Rueda3 and Cristina Garc´ıa-Ru´ız3

*Corresponding author: [email protected] 1Universidad de Málaga. Facultad de Ciéncias.Bulevar Louis Pasteur. Málaga 2Institut de Ciènciesdel Mar (CSIC). Passeig Mar´ıtimde la Barceloneta 37. Barcelona 3Instituto Español de Oceanograf´ıa,C.O. Málaga.Muelle pesquero s/n. Fuengirola

Keywords: Alboran Sea, Nephrops norvegicus, Parapenaeus longirostris, Population structure

Introduction

The deep-water shrimp Parapenaeus longirostris (Lucas, 1846) and the Norway lobster Nephrops norvegicus (Linnaeus, 1758) are two decapod crustacean species especially targeted by trawl ﬁsheries in the northern Alboran Sea and Gulf of Vera (western Mediterranean). Parapenaeus longirostris represents around 30% in weight and 20% in economic value of the total crustacean trawl landings in the area, while N. norvegicus represents around 4% in both, weight and economic value (Junta de Andaluc´ıaand Gobierno de Murcia, statistics 2017). The aim of this study is to analyze the spatio-temporal distribution and size structure of the P. longirostris and N. norvegicus populations along the northern Alboran Sea, including the Alboran Island, and the Gulf of Vera. This information is of importance for an appropriate management of ﬁsheries resources in order to achieve a sustainable exploitation.

Material and methods

The study area comprises: (i) the northern Alboran Sea (between Punta Europa and Cape Gata), (ii) the Gulf of Vera (between Cape Gata and Cape Palos) and (iii) the Alboran Island (Figure1).

Figure 1: Map of the study area showing the spatial distribution and abundance (ind km−2) of Parapenaeus longirostris (A) and Nephrops norvegicus (B). Hauls with absence of both decapods are plotted by X.

Data were obtained from 413 hauls performed from 2012 to 2018 under the framework of the European

1 Union research project MEDITS (International Mediterranean bottom trawl survey) which has been con- ducted annually (in spring). A stratified random sampling design was used with five bathymetric strata: 10-50 (A), 51-100 (B), 101-200 (C), 201-500 (D) and 501-800 (E). The gear used was a GOC 73 (mean vertical opening: 2.42 m, mean horizontal opening: 18.07 m, cod-end mesh size: 20 mm). The mean towing speed was 3 knots and the trawling time was 30-minute for depths <200 m and 60-minute for depths >200 m. Total catch for both species was separated and quantified by sex and, for each sex, the carapace length (CL) in mm was also recorded. The abundance (expressed as a number of individuals) of N. norvegicus and P. longirostris were standardized to 1 km2 (ind km−2) and plotted by year, depth and geographical area. Length-frequency distributions were constructed per 1-mm CL size class for each decapod species and plotted by sex, area, depth and year. The non parametric Kruskal-wallis test was used to test spatial and temporal differences in the mean abundance between depth strata and geographical areas. Parametric tests (Chi-square and Student-t) were used to analyze the spatial and temporal variation of the mean length of each species by depth strata, geographical area, year and sex. The sex-ratio was also calculated and compared by depth strata. Statistic tests were carried out with Statistica software. Statistic comparisons by depth strata were only carried in those strata where the species frequently occur (for P. longirostris B,C,D, E depth strata and for N. norvegicus D and E).

Results and discussion

Parapenaeus longirostris and N. norvegicus were distributed along the study area between 61-711 and 148- 793 m depth, respectively. Both species were caught in 30% of hauls and its occurrence was higher within the depth stratum D (200-500 m) (83% of hauls in stratum D for P. longirostris and 64% for N. norvegicus) (Figure1). In both decapods, significant differences for the mean abundance were detected in relation to depth (Kruskall-Wallis, p>0.05), with the highest abundance in depth strata D. Differences of the mean abundance between the three areas were not significant. Inter-annual variation of the mean abundance for P. longirostris was more acute, than for N. norvegicus, and significant fluctuations were detected in both species along the time series (2012-2018) (Figure2).

Figure 2: Annual abundance (ind km−2) for Parapenaeus longirostris (a) and Nephrops norvegicus (b) from annual MEDITS surveys (2012-2018) in the study area.

The overall length-frequency distribution of P. longirostris in spring was bimodal while for N. norvegicus it was polymodal. Sizes ranged between 10-40 mm CL for P. longirostris and 16-69 mm CL for N. norvegicus (Figure3). For P. longirostris, there are significant differences in the mean length between depth strata, geographical areas, years and sexes (t-test, p<0.05). Individuals from the deepest strata were significantly larger than those from the shallow strata. Regarding geographic areas, Alboran Island harbored the smallest individuals and the Gulf of Vera harbored the largest ones. Annually the mean length decreased with mean abundance increases. Regarding sexes, females were significantly larger (mean CL= 25.25 mm) than males (mean CL=22.71 mm) (t-test, p<0.05). Both sexes were equally represented but sex-ratio showed significant differences according to depth strata (X2, p<0.05), being females more frequent than males across all strata,

2 except in D stratum. For N. norvegicus the mean length was also higher in the deepest stratum (E) (t- test, p<0.05). Significant differences of the mean length were found in different geographical areas (t-test, p<0.05), with the largest individuals occurring in the Alboran Island and the smallest ones in Gulf of Vera. Regarding sexes, females were significantly smaller (mean CL= 38.64 mm) than males (mean CL= 43.20mm) (t-test, p<0.05). The sex ratio was slightly dominated by the females. Nephrops norvegicus also showed significant differences according to the depth strata (X2, p<0.05) as found for P. longirostris. Females were more abundant in the D stratum while males were more abundant in the deepest stratum (E).

Figure 3: Parapenaeus longirostris (a) and Nephrops norvegicus (b) length frequency distributions according to sex, obtained from MEDTIS surveys (2012-2018) in the study area. n: number of specimens measured.

As detected in previous studies, P. longirostris was more common and abundant in shallower strata (generally between 100-200 m depth) than N. norvegicus (generally below 500 m depth)(Abellóet al., 1988; Abellóet al., 2002). Inter-annual variations of the mean abundances for both species were similar to those of others studies in adjacent areas or to commercial landings trends (Sbrana et al., 2019). The differences between species are mainly due to their different life histories and phylogenetic constraints, since they belong to very different branches of decapod crustaceans. Different fishing effort rates and the interaction of environmental conditions with the biological characteristics of the species contribute to reshape their population traits. Both decapods showed sexual dimorphism in size with very similar ranges to those found in other studies: lower size of males for P. longirostris and of females for N. norvegicus (Garc´ıa-Rodr´ıguez et al., 2009). Additionally, both species also showed a marked, size-dependent distribution with depth, with small individuals generally occurring at shallower waters. The nearly complete absence of P. longirostris in shallow waters in the Alboran Island and the geographical differences in mean abundances of both species can be explained by different environmental conditions, bottom characteristics, prey availability or fishing pressure among the areas which may need further studies.

Acknowledgments

We wish to thank all participants in the MEDITS project. And thanks to Josetxu Ortiz de Urbina for his suggestion in the statistical analysis.

References

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