Environmentally driven synchronies of Mediterranean cephalopod populations

Highlights

• Synchronies in population dynamics of Mediterranean cephalopod species are revealed.

• A general increasing trend in cephalopod abundances is found.

• Temperature and productivity affect population dynamics on a regional scale.

• Populations are more closely connected in the Eastern than in the Western basin.

• Results are relevant for current European management regulations.

Abstract

The Mediterranean Sea is characterized by large scale gradients of temperature, productivity and salinity, in addition to pronounced mesoscale differences. Such a heterogeneous system is expected to shape the population dynamics of marine species. On the other hand, prevailing environmental and climatic conditions at whole basin scale may force spatially distant populations to fluctuate in synchrony. Cephalopods are excellent case studies to test these hypotheses owing to their high sensitivity to environmental conditions. Data of two cephalopod species with contrasting life histories (benthic octopus vs nectobenthic squid), obtained from scientific surveys carried out throughout the Mediterranean during the last 20 years were analyzed. The objectives of this study and the methods used to achieve them (in parentheses) were: (i) to investigate synchronies in spatially separated populations (decorrelation analysis); (ii) detect underlying common abundance trends over distant regions (dynamic factor analysis, DFA); and (iii) analyse putative influences of key environmental drivers such as productivity and sea surface temperature on the population dynamics at regional scale (general linear models, GLM). In accordance with their contrasting spatial mobility, the distance from where synchrony could no longer be detected (decorrelation scale) was higher in squid than in octopus (349 vs 217 km); for comparison, the maximum distance between locations was 2620 km. The DFA revealed a general increasing trend in the abundance of both species in most areas, which agrees with the already reported worldwide proliferation of cephalopods. DFA results also showed that population dynamics are more similar in the eastern than in the western Mediterranean basin. According to the GLM models, cephalopod populations were negatively affected by productivity, which would be explained by an increase of competition and predation by fishes. While warmer years coincided with declining octopus numbers, areas of high sea surface temperature showed higher densities of squid. Our results are relevant for regional fisheries management and demonstrate that the regionalisation objectives envisaged under the new Common Fishery Policy may not be adequate for Mediterranean cephalopod stocks.

Introduction

The Mediterranean Sea is generally shaped by large scale gradients of temperature, productivity and salinity (D’Ortenzio and Ribera d’Alcalà, 2009). However, it is also known as a complex ecosystem with contrasting regions in terms of productivity (Nieblas et al., 2014), seafloor topography and hydrography (Millot, 2005, Rossi et al., 2014). In an ecosystem with such pronounced regional differences, animal populations are prone to exhibit patchy distributions due to different habitat conditions. Such patchiness has been observed analysing feeding habitats of high trophic level species such as whales and tuna (Druon et al., 2011, Druon et al., 2012, Druon et al., 2016), the distribution of secondary consumers such as cephalopods or small elasmobranchs (Puerta et al., 2015, Puerta et al., 2016a, Navarro et al., 2016) or nursery grounds and spawning areas of commercially important species (Druon et al., 2011, Druon et al., 2015, Colloca et al., 2015). All these studies revealed that hydrographic conditions (Druon et al., 2011, Druon et al., 2015), bathymetric features like depth and type of seafloor (Druon et al., 2012, Colloca et al., 2015) and productivity (Druon et al., 2011, Druon et al., 2012, Puerta et al., 2016a) are key drivers of distribution patterns.

Environmental and large-scale climatic variability may force spatially distant populations to fluctuate in synchrony (Liebhold et al., 2004). Comparing the dynamics of spatially distant populations allows detecting the main drivers of abundance fluctuations and the scale at which they operate (Descamps et al., 2013), and thus the degree of connectivity of neighbouring populations. In synchronized populations, drastic declines in population size likely affect all populations simultaneously, exposing them to a greater risk of extinction (Descamps et al., 2013 and references therein). By contrast, heterogeneous and complex populations generally show higher resilience and recovery rates and are more likely to withstand mass elimination (Schindler et al., 2010). Therefore, knowledge about the underlying mechanisms of population dynamics at large spatial scales has important implications for the management and conservation of species (Liebhold et al., 2004).

Large-scale synchronic fluctuations have been described in phytoplankton (Doyle and Poore, 1974), zooplankton (Batchelder et al., 2012), fish (Kelly et al., 2009) and crustaceans (Koeller et al., 2009, Hidalgo et al., 2015). However, very little is known about cephalopods. This group is considered excellent for case studies to analyse synchronic fluctuations, owing to its sensitivity to changing environmental conditions as a result of its fast growth rates and short life cycles (Pierce et al., 2008). Among the environmental parameters investigated until now, sea surface temperature (SST) plays an important role in driving cephalopod abundance trends (Zuur and Pierce, 2004, Chen, 2010). Additionally, a recent study investigated local scale seasonal synchronies in Mediterranean cephalopod abundances (Puerta et al., 2016b), revealing contrasting seasonal cycles in response to local environmental conditions.

Due to the decline in the traditional finfish resources, European cephalopod fisheries, which were once located mostly in the Mediterranean, have grown and expanded northwards (Pierce et al., 2010). This expansion is expected to continue as a result of the increasing market demand of cephalopods, whereby regular assessments and certain management measures would be needed to ensure a sustainable exploitation. Under the current Common Fisheries Policy (CFP; EU Regulation N° 1380/2013), the regionalization approach may lead to a revision of the management areas currently in place in the Mediterranean (see below). For a successful implementation, gaining knowledge of the mechanisms controlling stock fluctuations is essential, as defining management areas merging independently fluctuating populations may result in strong miss-management. Sound scientific knowledge is necessary to implement management areas appropriately matching the biological distribution of populations as demanded by the new CFP in their regionalised ecosystem-based approach.

Here we analyse the population dynamics of two commercially important cephalopods (Sartor et al., 1998, Quetglas et al., 2000) with contrasting life histories, the nectobenthic broadtailed shortfin squid Illex coindetii and the benthic common octopus Octopus vulgaris, in the whole Mediterranean Sea. The data analysed were obtained from scientific surveys carried out throughout the Mediterranean during the last 20 years. The objectives of this study are threefold. For each species, we first analyse the global correlation structure across the entire Mediterranean Sea to assess how the similarity in population abundances of spatially separated populations (or population sub-units) decreases with distance. Second, we use dynamic factor analysis to detect underlying common abundance trends over distant regions. Finally, we analyse putative influences of key environmental drivers (productivity and SST) on the population dynamics at regional scale using general linear models.