Abstract
Without baseline data from near pristine assemblages, measures of ecosystem change may be significantly underestimated. A unique historical dataset provided an opportunity to investigate long-term change in demersal fish assemblages of South Africa's inshore trawl grounds. Three sites surveyed over a period from 1903 to 1904 were re-surveyed in 2015 using replicated historical gear and methods. Catch composition was contrasted between historical and modern periods using unconstrained ordination, permutational multivariate analysis of variance, permutational tests of the homogeneity of multivariate group dispersions and similarity percentage analyses. After 111 years, the re-survey revealed a drastically transformed demersal assemblage, with the period effect explaining nearly half of the variance among samples. Historical catches were dominated by kob (Argyrosomus spp.), panga (Pterogymnus laniarius) and east coast sole (Austroglossus pectoralis), jointly contributing 70-84% of the catch. The same taxa made up a minor component (1.5-5.5%) of modern assemblages. Instead, the re-survey catches consisted predominantly of gurnards (Chelidonichthys spp.), Cape horse mackerel (Trachurus capensis), spiny dogfish (Squalus spp.), shallow-water hake (Merluccius capensis) and white sea catfish (Galeichthys feliceps), with their summed contribution rising to 85% from the historical 3%. These results suggest that a century of trawling may have altered benthic habitats, indirectly contributing to changes in the fish community. Historical assemblages included a substantial proportion of taxa that associate with reef habitats, whereas the re-survey assemblages were characterised by species that inhabit unconsolidated sediments or both reef and non-reef habitats. The unique historical context and data, comparable gear and methods and long temporal period revealed striking baseline changes that may be overlooked in most fisheries. Reconstructing this important historical context improves our ability to assess, interpret and manage changing marine ecosystems.
Highlights
Effective policy advice on the management of biodiversity, fisheries and the ecosystems that support them requires information about the scale and drivers of past changes in the ocean (Schwerdtner Máñez et al, 2014; Engelhard et al, 2016)
Quantitative contrasts between the historical records and more recent survey data are challenging due to the substantial changes in trawling technology over time (Rijnsdorp et al, 1996; Rogers and Ellis, 2000; Mussgnug, 2013). We addressed this challenge by replicating the trawl surveys conducted by the Pieter Faure survey vessel more than a century previously
Trawl samples were selected for inclusion in analyses based on the following criteria: (1) they included a start and end time or trawl duration, of which the latter was limited to
Summary
Effective policy advice on the management of biodiversity, fisheries and the ecosystems that support them requires information about the scale and drivers of past changes in the ocean (Schwerdtner Máñez et al, 2014; Engelhard et al, 2016). Without knowledge of previous change and baseline reference points, recent variability within a fishery or ecosystem may provide little context for interpretation (Thurstan et al, 2010; Moloney et al, 2013), which hinders informed decision making. Anthropogenic impacts such as fishing and climate change have altered marine ecosystems (Jackson et al, 2001; Lotze et al, 2006; Brander, 2010; Cheung et al, 2013; Moloney et al, 2013). Knowledge of near pristine ecosystems and the reference points that describe them are broadly lacking, both in South Africa and across most of the globe
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