Assessment of the impact of gillnets on the population structure of the shovelnose guitarfish Rhinobatos productus from the Gulf of California, Mexico

The shovelnose guitarfish Rhinobatos productus is the most abundant species in the artisanal ray fishery of the Gulf of California. This ray is captured when it approaches the coast for reproductive purposes and is highly vulnerable to gillnets used in the fishery. Despite the high production of elasmobranchs in the Gulf of California, studies on reproductive and technological aspects are scarce. In the present study, we analyzed the interaction between size-at-maturity and size-selectivity, which are represented by a logistic function and gamma distribution, respectively. Interaction of such vectors is applied to the size-frequency distribution of the population monitored in Sonora (Mexico) during 1998–2004. The effect of maturity ogives revealed a proportion of mature individuals of 68% and 83% for males and females, respectively. Gillnet selectivities with different mesh sizes had a varied effect on the size structure given that males reach smaller sizes than females. For males, selectivity curves of nets with mesh sizes of 3.5, 5.0, 6.0, 8.0, and 8.5 inches reduced the original length-frequency distribution by 59%, 9%, 8%, 85%, and 94%, respectively. The same nets reduced the original length-frequency distribution of females by 79%, 41%, 16%, 48%, and 63%. Gillnets with mesh sizes of 5.0 and 6.0 inches had higher selectivity for both sexes coinciding in the size interval of maximum abundance. The results of the present study revealed a negative impact of the fishery on the adult structure of the population and to a lesser extent on juveniles. A demographic analysis of this population incorporating both maturity and selectivity vectors will allow determining the resilience to the current exploitation pattern by elasticity analysis.

Size at maturity of the Pacific sharpnose shark Rhizoprionodon longurio in the Gulf of California, Mexico

We report on the proportion of maturity stages as a function of size of the Pacific sharpnose shark Rhizoprionodon longurio. Based on the development of internal and external organs, males and females were classified as immature or mature. A logistic model was fitted to develop a maturity curve to estimate the length at which 50% of the individuals are mature (L50%). This species has an annual reproductive cycle and its birthing season is from April to July, with June as the main month. For males, L50% = 100.61 cm total length (TL) and Φ = 2.57, and for females, L50% = 92.9 cm TL and Φ = 1.08. For pregnant females, L50% = 106.4 cm TL and Φ = 6.11. The maturity curves obtained in the present study can be used in demographic studies to produce more accurate forecasts of the population reproductive rate and thus develop fishery management measures for this species