Stock assessment of the Spanish mackerel (Scomberomorus commerson) in Saudi waters of the Arabian Gulf with generalized depletion models under data-limited conditions

A multi-annual and multi-fleet version of a generalized depletion model is developed and applied to the Spanish mackerel (Scomberomorus commerson) stock in Saudi waters of the Arabian Gulf under data-limited conditions. The fishing operational data are the official monthly records of estimated catch biomass and exact nominal effort of two artisanal fleets from 1995 to 2012. To transform catch in biomass to catch in numbers, a monthly trend in mean weight is estimated from biological compositional data from 2002, 2013 and used to produce predictions of mean weight for every month between 1995 and 2012. Estimates of natural mortality and initial abundance, thirty six fleet-specific abundance parameters, and six fleet-specific fishing operational parameters, are obtained by maximum likelihood. Thirty six additional timing parameters are fixed by graphical display of catch spikes, a convenient statistic defined here, and by maximum likelihood. The reconstructed history of the stock shows that the catch varies in direct relation to the magnitude of recruitment pulses, while nominal effort has been stable. The annual exploitation rate was estimated at approximately 10%, which implies that fishing is occurring at sustainable levels, although these results are conditional on a well-behaved mean monthly weight in the catch and simplified statistical models.