The systematic degradation of marine ecosystems is a global phenomenon that has important consequences including biodiversity loss and reduced ecosystem service provisions. In temperate subtidal regions, kelp forests dominate rocky coasts, and the abundance and resilience of these forests are influenced by local-scale stressors and regional environmental variation. Fisheries is one of the main factors affecting the abundance and persistence of marine ecosystems. Fisheries can directly affect the abundance and population structure of exploited resources (and its associated species), yet it may indirectly affect species interacciones. One clear example of ecosystems affected by direct and indirect effects of fisheries are kelp along the humboldt current system, were fishes and invertebrates inhabiting kelp forests and the kelp forest itself are being degraded by harvesting kelp individuals, an activity localy known as “barreteo”. However, limited information exists regarding the direct and indirect consequences of kelp harvesting which, in turn, limits the ability to provide effective management and conservation efforts. In this thesis I examined the direct and indirect consequences of the artisanal subtidal Lessonia trabeculata fishery and the kelp community of benthic species associated with it along the coast of central Chile.
During a two-year period, I assessed the community level consequences of the direct harvesting of kelp Lessonia trabeculata. In this effort, I assessed (a) the change in adult and juvenile Lessonia trabeculata density as a consequence of harvesting, (b) the impact of Lessonia trabeculata harvesting on reef fish, mobile macroinvertebrate and sessile species assemblages, and (c) the change in density of the most abundant Lessonia trabeculata -associated species. The experiment was conducted from december 2016 to may 2019. In total, after removal of kelp, the size of each plot was ~90m2, and ~90% of Lessonia trabeculata was removed by an experienced kelp fisherman. I used band transects and quadrats to estimate the density of reef fish, mobile macroinvertebrates and the % coverage of sessile species through periodic monitoring previous to and after kelp harvesting in control and harvested plots. The results documented the effects of kelp harvesting by artisanal fishers at a multi-trophic level (from algaes to secondary predators). Even after two years post-harvest, Lessonia trabeculata still shows no signs of recovery. The results indicate that a single, once-off harvesting disturbance from the kelp forest resulted in significant alteration of local reef fish, macrobenthic invertebrate and sessile species assemblages. Previous to harvesting no differences were observed between the assemblages of control and harvested plots. After harvest, within the harvested plots, the average abundance of the fish Scartichthys viridis, the rock shrimp Rhynchocinetes typus and the herbivorous snail Tegula tridentata was greater than non-harvested control plots. On the contrary, a greater abundance in the control plots was observed for the fishes Aplodactylus punctatus, Isacia conceptionis and Chromis crusma. This study also allowed us to evaluate the natural changes in kelp recruitment (i.e. newly settled individuals that have survived one and two years after kelp harvesting, sensu Rodriguez et al., 1993). The difference in abundance of associated species may be key to the lack of recovery of Lessonia trabeculata forests. The study highlights the impact of Lessonia trabeculata harvesting on associated fauna, however, significant knowledge gaps remain, particularly with respect to whether Lessonia trabeculata will fully recover after harvesting and if so, how long will it take to re-establish its original biomass, as well as its associated fauna at the scale at which the fisheries operate.
A second set of studies were developed to evaluate the influence of changes in Lessonia trabeculata kelp forest, mediated by the direct and indirect effects of fisheries, on the recruitment patterns of benthic species. Fisheries can affect kelp forests directly, by altering the density of the forest (i.e. kelp thinning), or indirectly, by removing top predators and altering trophic interactions and ultimately affecting grazing activity on kelp which consequently alters the frondosity of the kelp canopy and ultimately alters the kelps forest complexity. The studies focus on both (a) the direct influence of kelp harvesting and (b) the indirect effect of artisanal fisheries on kelp forest morphology and its consequential effect on the recruitment patterns of benthic species in central Chile. To address the direct effect of Lessonia trabeculata kelp harvesting, recruitment assemblages and recruitment rates of benthic invertebrates were compared between harvested (almost complete kelp removal) and control plots, taking advantage of a large-scale experiment described in the aforementioned study. However, the consequences of harvesting are not necessarily the complete removal of a forest rather the thinning of the density of the kelp forest. Thinning is an expected output of regulated kelp fisheries, since the state suggest to harvest plants sparsely. Thus, the second specific goal evaluated the recruitment patterns (recruitment assemblage and recruitment rate) of benthic invertebrates among three densities of kelp: zero, medium and high. Finally, I assesed the indirect effect of fisheries on recruitment patterns, mediated by changes in kelp morphology. Extense defoliated patches of Lessonia trabeculata have been reported, presumably related to an indirect effect of the depletion of top predators due to coastal fisheries, which ultimately free a top down pressure on herbivores causing a cascading effect that alters the kelp morphology, and reduces its the kelp forests structural complexity. Recruitment assemblage and recruitment rate of benthic invertebrates was evaluated comparing plots of defoliated kelp (no canopy formed, i.e. a “skeleton” morph) and completely foliated natural plots of kelps. Since reproduction of most benthic species in temperate regions is seasonal and hence recruitment, in a first study a full annual cycle was sampled, with one sampling time in each season (autumn, winter, spring and summer) and thus determined the season with greater recruitment of benthic species, and later only evaluated the recruitment in one season for the other two studies, i.e. the austral summer. This set of studies focus exclusively on patterns of recruitment, as a first step to understand the consequences of fisheries. It does not pretend to investigate the processes involved. I used standard monitoring units known as SMURF (standard monituring unit for the recruitment of fish adn invertebrates) to evaluate recruitment. This standard monitorng units were placed in every treatment and later retrieved every couple of weeks. Once in the lab individual recruits where identified to the lowest taxonomical level possible. Recruitment rate of each taxa was measured by dividing the number of individuals per taxa by the number of days that the SMURF were deployed in the field. The results of the studies conducted to assess the direct and indirect effect of fishing activities on Lessonia trabeculata kelp forests showed that (a) kelp harvesting, (b) kelp thinning and (c) kelp morphology affects the recruitment assemblage and the recruitment rate of benthic species. Over 50 taxa were identified and although, the studies were conducted at different sites along the coast of central Chile, and at different time frames, only five species were persistently the most abundant: Eulithidium umbilicata, Pilumnoides perlatus, Taliepus sp., Tetrapygus niger and Semimytilus algosus. These taxa all together represented over 50% of the total number of recruits registered. The results show three general patterns: (a) an effect of the experimental treatments on recruitment assemblage and recruitment rate, reflecting the direct and indirect influence of fisheries on kelp forest complexity and the consequent effect on recruitment assemblage and recruitment rate, (b) different responses in the different studies and species of recruits. These results suggest a need to advance in the management of Lessonia trabeculata fisheries to better account for ecosystem-wide impacts in order to better manage and protect this vital coastal ecosystem.