<b>Unveiling Ecosystem Shifts in the southern Benguela through otolith biochronologies of sardine (</b><b><i>Sardinops sagax</i></b><b><i>)</i></b>

Sardines (<i>Sardinops sagax)</i> of the southern Benguela have shown substantial changes in population size over the past 70 years. A large population that initially supported high catches collapsed in the early 1960s, recovered in the 1990s and was again large in the early-2000s, and then collapsed again, the last collapse associated with a change in distribution. We developed a robust otolith biochronology of annual increment width to infer long-term growth patterns of sardine using 849 otoliths archived in South Africa from 1966 to 2019. All archived otoliths used were sourced from the Department of Forestry, Fisheries and the Environment (DFFE) in South Africa. Moreover, only archived otoliths originating from sardines caught west of Cape Agulhas were used for increment analysis. The archived otoliths stems from DFFE annual surveys in November as well as commercial port samples. The annual November hydro-acoustic pelagic biomass survey was conducted on board FRS Africana and covered the entire shelf area between Hondeklip Bay on the west coast and Port Alfred on the southeast coast. Commercial port otolith samples were taken from ports sampled west of Cape Agulhas off commercial purse-seine vessels. Thus, events are titled according to the transect station of the hydro-acoustic biomass survey (e.g. 29-05A) or the sampling port station from commercial catches (e.g. Gansbaai).Biological measurement obtained from archived otolith (if available) were recorded such as total lengths (TL ± 0.01 mm), month, year and station number. The sagittal otoliths were mounted whole in black Perspex plate moulds and embedded in epoxy resin. Calibrated images of the sardine otoliths were taken using Zeiss Stemi 508 stereo microscope that is fitted with a Carl Zeiss Axiocam 208 colour camera. In order to assess the age of sardine at a given length, otolith annual growth increments were counted (core to edge) along the otolith's largest radius using a Zeiss Stemi 508 stereo microscope 320× magnification. We analysed a total of 787 otolith images using ObjectJ which is an ImageJ plugin (http://imagej.nih.gov/ij/). The length of each annual increment on the along the post-rostrum axis of the otolith from the nucleus to posterior edge.<br>Linear mixed effect models were applied to separate intrinsic (within individual) and extrinsic (by year – e.g. population density, environment) factors to estimate mean annual growth for fish older than 1 year (&gt;1 years) refer to table with sardine predicted growth. The increment widths were recorded together with its year when the increment was formed (calendar year), and age at the time of increment formation (Age) for each fish (refer to table with otolith incremental data).Project(s): TRIATLAS (Tropical and South Atlantic Climate-based Marine Ecosystem Prediction for Sustainable Management)