Relation between the sharpnose shark Rhizoprinodon terranovae in the southern Gulf of Mexico and the average number of sunspots

Background. The Antillean or sharpnose shark is one of the main exemplars of shark species in the commercial catch in the southern Gulf of Mexico. Oceanic dynamics on the planet Earth vary year to year due to the oscillation of solar radiation; fish abundance (and catches) likely respond to these oscillations. Goals. This study focuses on analyzing the variability of landings of the Antillean sharpnose shark (Rhizoprionodon terraenovae) on the Mexican coast of the Gulf of Mexico, from 1940 to 2006, and its possible connection with solar activity as assessed by the average sunspot number (Zurich Index). Methods. A polynomial curve by least-squares best fitting was used to eliminate the tendency from the sharpnose-shark-landing time series. Spectral techniques of Maximum Entropy (ME) and Fast Fourier Transform (FFT) were used to analyze the time series of sharpnose-shark landings and the Zurich Index. Results. The time series of sharpnose-shark landings showed a remarkable spectral peak with a period of about 12.8 years, while the Zurich Index showed a marked peak at a period of 11.6 years. Cross-correlation revealed a delay between the average number of solar spots and sharpnose-shark landings. Conclusions. This study is the first to report such low frequency variability for landings of the R. terraenovae fishery in the Gulf of Mexico, using a particular methodology that can provide information for the design of predictive models in the future. The analyzed landing volumes of the sharpnose shark R. terraenovae showed as a result a periodical behavior that coincided with Wolf's solar activity cycle of 10.6 years.