Distribution and spatio-temporal biomass trends of red mullets across the Mediterranean

Abstract

The present work examines the spatio-temporal biomass trends of Mullus barbatus and Mullus surmuletus in the Mediterranean Sea through the analysis of a time series of data coming from the Mediterranean International Trawl Surveys (MEDITS), accomplished annually from 1994 to 2015. The biomass of both species showed clear declining trends below 150 to 200 m depth, which were steeper in the case of M. barbatus. Increases in temporal biomass trends were observed for M. barbatus from 2008 onward in most geographic sub-areas (GSAs), while stability was mostly observed for M. surmuletus. For both species, dynamic factor analysis revealed similarities among neighbouring GSAs and the subsequent cluster analysis identified two major GSA groups corresponding to the eastern and western basins of the Mediterranean. Overall, the results suggested that the combined effects of fishing and environmental conditions determine species abundance variations, but the relative importance of each component may vary among areas.

Copernicus Marine Service Ocean State Report, Issue 5

“…For example, results from Sion et al (2019) on European hake show for 2011 and 2013 higher biomass values in the eastern-central Adriatic sea, while in 2015 a general lower biomass of this species was estimated, with similar outcomes to the ones we found in this paper (Figure 3.6.2). Tserpes et al (2019) also highlights a biomass increasing trend for red mullet after 2008, which is in line with the recent stock assessment outcomes (GFCM 2019; STECF 2019). Similarly, Figure 3.6.2 highlights that this biomass increase corresponds to a spreading of the population in the study area.…”

“…Furthermore, the Delta-GAM approach was implemented in two steps: (i) a binomial occurrence model was used to fit presence/absence data (binomial family error distribution logit link function), (ii) a Gaussian distribution model with identity link function on transformed biomass for presence-only data (Grüss et al 2014;Lauria et al 2017). A grid of regular points with the same resolution of the selected CMEMS product (1/16°) and covering the study area was created to predict species biomass distribution by the selected models (Tserpes et al 2019;Spedicato, Zupa, et al 2019). For Delta-GAM the final spatial distribution of species biomass as kg/km 2 is obtained by multiplication of Gaussian and Binomial models' predictions to the grid of the model's domain (Grüss et al 2014;Lauria et al 2017).…”

“…In order to test the potential benefits on using oceanographic and effort variables in addition to spatiotemporal covariates (latitute, longitude, depth, year and month) to improve species distribution models based on trawl survey data, Generalised Additive Models (GAMs) were chosen for their wide application and suitability with trawl survey data (Grüss et al 2014;Lauria et al 2017;Tserpes et al 2019). GAMs allow to predict species abundance and biomass over the domain (Maunder and Punt 2004;Rubec et al 2016;Potts and Rose 2018) and provide estimates useful for tuning stock assessment models (Cao et al 2017;Orio et al 2017).…”

“…The relative high number and the quality of the CMEMS products, as well as their high temporal coverage and spatial resolution, provide biogeochemical and physical oceanographic variables that can be useful to improve the analysis of abundance and biomass indices derived from trawl surveys (e.g. Sion et al 2019;Tserpes et al 2019).…”

Declining population sizes and loss of genetic diversity in commercial fishes: a simple method for a first diagnostic

“…However, the differences in the levels of COI-π between the two Mullus species may also be related to different habitat preferences. M. surmuletus prefers narrow shelf areas with rocky substrates while M. barbatus is more abundant in areas where shelf becomes wider with muddy bottoms (Lombarte et al, 2000;Tserpes et al, 2019). Because rocky areas are not accessible by bottom trawling, a significant proportion of the M. surmuletus population is not directly affected by this threat.…”