School of BioSciences - Theses
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ItemEffects of fishing and climate change on the Chondrichthyan species in the Gulf of California region( 2019)This thesis is composed of three data chapters and a general introduction and discussion. Each chapter, except for the general introduction and the general discussion is composed of an introduction, materials and methods, results, discussion, and conclusions. The aim of this study was to assess the vulnerability risk to fishing and climate change stressors of 106 species of chondrichthyans with ≥10% of their distribution within the EEZ off western Mexico. For my analysis, I determine the vulnerability of the chondrichthyan species inside the Gulf (GCI) and compare these results with those for two other contiguous broad regions with different oceanographic conditions, the region around the entrance to the Gulf of California (GCE) and Mexico’s remaining Pacific waters (MPW). I have built on existing approaches to provide, in a single framework, a vulnerability analysis and risk assessment of the Mexican chondrichthyan (sharks, rays, skates and chimaeras) fauna by combining three components of vulnerability risk to climate change (exposure, sensitivity and adaptive capacity) (ESA), together with three components of vulnerability risk to fishing stressors (exposure, productivity, and susceptibility) (EPS). Here, vulnerability is expressed as the risk of marked reduction of the population of chondrichthyans based on the knowledge of its biology and its exposure to stressors associated with fishing and climate change. For fishing stressors, I use the productivity of the chondrichthyan species, which is related to the maximum age of the species, and susceptibility, which derives from four parameters; availability, encounterability, selectivity, and post-encounter mortality. For climate change stressors, I use sensitivity, which has two parameters; rarity and habitat specificity as species attributes that contribute to this, and adaptive capacity. Adaptive capacity involves distributional flexibility and trophic level as relevant attributes. I assigned each species to one of six ecological groups (EGs), which is a flexible and novel way to allocate a large number of species based on habitat use, depth strata (shelf-inshore and shelf-offshore), habitat dependence (freshwater, reef substrate, and sandy substrate), and lifestyle (demersal or pelagic). For fishing stressors, I analyzed data sets from 2006 to 2017 for the prawn trawl fishery, the elasmobranch fishery (artisanal and semi-industrial) and for the sardine fishery, and published information on the sport-recreational fishing. These fisheries have the potential to reduce the size of the population of a chondrichthyan species by altering the mortality rate in the regions where the fisheries operate. I then characterised the fishing stressors in terms of fishing methods and the bathymetric range of deployment of the fishing gear (Chapters 1, 3 and 4). For climate change stressors, I obtained data sets from several sources to show trends in the past oceanographic conditions and how they may vary in response to climate change. I then characterised the oceanography of the Gulf of California and adjacent waters. My assessment is based on observed changes from 1960 to 2017, and projected changes. In the period from 1960 to 2017 two important phenomena that warm the sea surface water occurred; “El Nino” and “El Blob”. The latter is a phenomenon related to a warm mass of water as a result of high levels of atmospheric pressure and of which origin is detected in the Gulf of Alaska in 2013. The name “Blob” echos the 1958 horror film which describes a character that keeps growing as it consumes everything in its path just as this warming event did (Cornwall, 2019). The “El Blob” was detected until several months later 2013, so it is unknown whether “El Blob” can occur again with the same or higher intensity. The other timescales are based on projected changes by 2055 and by 2099 using low (2.6), medium (4.5) and high (8.5) emissions scenarios from the RCP (representative concentration pathway) family. Because of a temperature gradient in coastal waters increasing from north-western Mexico to southwestern Mexico (Chapter 2), I established ten contiguous ‘subregions’ in these waters (Chapter 4). This allowed me to evaluate the risk associated with the attribute 'distributional flexibility' of the chondrichthyan species and to determine thermal tolerance range categories as follows: all waters (AW), cool waters (CW), warm waters (WW) or Gulf of California water (GoCw). These categories provided a basis for projecting how chondrichthyan distributions might change in response to climate change. I identified a total of 54 species of sharks, 48 species of rays and 4 species of chimaeras, which belong to 3 superorders, 12 orders, and 33 families. Based on the thermal tolerance range indicated by the current presence-absence of each species in the subregions, a total of 35 chondrichthyan species are distributed in all Mexican waters (AW), suggesting the species are adapted to the full range of temperatures currently occurring in Mexican waters. The majority of these are commercial shark species and these are the least likely species to redistribute out of Mexican waters as waters warm progressively northward as climate change progresses. A total of 31 species were classed CW (i.e., favouring cooler waters) and likely to reduce their distributional range northwards as Mexican waters warm in response to climate change. The majority of these are also commercial shark species. On the other hand, 34 species of chondrichthyans were classed WW (i.e., favouring warm waters), and are likely to expand their distribution northwards within Mexican waters. The majority of these are ray species, some of them of commercial importance. One species of shark, one species of ray and one species of chimaera are distributed only in the GoC waters, and another species of shark and two species of rays are distributed in only inside and outside the GoC in the adjacent MP-C subregion. The ecological groups (developed for all three regions) are shelf-inshore, shelf-reef, shelf-sand (<75 m), shelf-sand (75–150 m), pelagic waters and bathyal (>150 m). A total of 46 species were allocated to the ecological group ‘shelf-sand (<75m)’, 14 species were allocated to the ecological group ‘shelf-sand (75–150 m)’, and 22 species to the ecological group ‘pelagic waters’. Some of these species are demersal and others swim near the bottom or may swim up in the water column. A total of 19 species of chondrichthyans are in the ecological group ‘bathyal (>150 m)’, one species is in the ecological group ‘shelfinshore’, and 4 species were allocated to the ecological group ‘shelf-reef’. Vulnerability risk varies among the current chondrichthyan species, among ecological groups and among fishing and climate change stressors. For total vulnerability to fishing stressors, there were 10 species in the GCI and GCE regions, and 40 species in the MPW region at medium vulnerability risk. I determined 33 species in the GCI and GCE regions, and one species in the MPW region were at high vulnerability risk. For climate change (CC) stressors in the whole of western Mexico, a total of 15 and 10 species were at medium vulnerability risk under the medium and high emissions scenarios, respectively, and 10 species were at high vulnerability risk under the high emission scenario. The species allocated in the EG shelf-sand (<75 m) are highly vulnerable to the combination of fishing and CC stressors in all three regions for all the CC scenarios. In contrast, the species allocated in the EG bathyal (>150 m) are at low vulnerability but varies for species allocated to the other EGs.