Population productivity of shovelnose rays: Inferring the potential for recovery

Abstract

There is recent evidence of widespread declines of shovelnose ray populations (Order Rhinopristiformes) in heavily fished regions. These declines, which are likely driven by high demand for their fins in Asian markets, raises concern about their risk of over-exploitation and extinction. Using life-history theory and incorporating uncertainty into a modified Euler-Lotka model, the maximum intrinsic rates of population increase (rmax) were estimated for nine species from four families of Rhinopristiformes, using four different natural mortality estimators. Estimates of mean rmax, across the different natural mortality methods, varied from 0.03 to 0.59 year-1 among the nine species, but generally increased with increasing maximum size. Comparing these estimates to rmax values for other species of chondrichthyans, the species Rhynchobatus australiae, Glaucostegus typus, and Glaucostegus cemiculus were relatively productive, while most species from Rhinobatidae and Trygonorrhinidae had relatively low rmax values. If the demand for their high-value products can be addressed then population recovery for some species is likely possible, but will vary depending on the species.

Fig 1. Incorporating uncertainty in the model parameters when predicting values of r_max (year^-1) for nine shovelnose rays species.

When including uncertainty in age at maturity (α_mat, first/orange boxplot), annual reproductive output (b, middle/blue boxplot), and reciprocal of the lifespan natural mortality estimator (M, last/grey boxplot). Species are (A) R. australiae, (B), G. cemiculus, (C) G. typus, (D) A. annulatus, (E) P. horkelii, (F) P. productus, (G) R. rhinobatos, (H) Z. brevirostris, and (I) Z. exasperata. Boxes indicate median, 25 and 75% quantiles, whereas the lines encompass 95% of the values (2.5 and 97.5% quantiles). For plots incorporating uncertainty with other natural mortality methods, see S2 Appendix.

Fig 2. Values of r_max (year^-1) for nine shovelnose ray species vary with different methods of estimating natural mortality.

Which are Jensen’s First Estimator (red), modified Hoeing & Hewitt’s Estimator (yellow), Frisk’s Estimator (green), and Reciprocal of lifespan (blue). Means (triangle) and standard deviation (black line) are presented for each method. Species are (A) R. australiae, (B), G. cemiculus, (C) G. typus, (D) A. annulatus, (E) P. horkelii, (F) P. productus, (G) R. rhinobatos, (H) Z. brevirostris, and (I) Z. exasperata. Values below the black dashed line indicate implausible r_max estimates.

Fig 3

Predicted value of r_max for the nine species of shovelnose rays in relation to their (A) age at maturity (a_mat, years) and (B) annual reproduction rate of females (b). The black lines encompass 95% of the values (2.5 and 97.5% quantiles). The reciprocal of lifespan natural mortality estimator to estimate r_max. The shapes represent the four families; black circles represents the giant guitarfishes, Family Glaucostegidae; black triangles signifies the wedgefishes, Family Rhinidae; black squares represents guitarfishes, Family Rhinobatidae; and black crosses are banjo rays, Family Trygonorrhinidae.

Fig 4

Maximum size(cm TL) for the nine species of shovelnose rays in relation to the (A) median maximum intrinsic rate of population increase (r_max, year^-1) using the reciprocal of lifespan to estimate natural mortality, (B) annual reproduction rate of females (b), and (C) size at birth (cm TL). The black lines encompass 95% of the values (2.5 and 97.5% quantiles). The shapes represent the four families; black circles represents the giant guitarfishes, Family Glaucostegidae; black triangles signifies the wedgefishes, Family Rhinidae; black squares represents guitarfishes, Family Rhinobatidae; and black crosses are banjo rays, Family Trygonorrhinidae.

Fig 5. The frequency of the r_max values predicted for 115 chondrichthyans, including the nine shovelnose ray species.

The reciprocal of lifespan natural mortality estimator was used to estimate r_max and species are grouped by their r_max values. Black line denote the mean (r_max = 0.30) and blue line represents the median (r_max = 0.23). The nine shovelnose rays species are displayed on the figure and species illustrations are from Last et al. [75].

Fig 6. Estimates of r_max for 115 chondrichthyans, including the nine shovelnose rays species, compared with life history parameters.

(A) maximum size (cm TL/DW), (B), age at maturity (α_mat years), (C) maximum age (α_max, years), (D) annual reproductive output b, (E) the von Bertanlaffy growth coefficient (k, year^-1). The nine shovelnose ray species labelled are: RA, R. australiae; GC, G. cemiculus; GT, G. typus; AA, A. annulatus; PH, P. horkelii; PP, P. productus; RR, R. rhinobatos, ZB, Z. brevirostris; ZE, Z. exasperata. The black lines encompass 95% of the values (2.5 and 97.5% quantiles). The median r_max value is reported, using the reciprocal of the lifespan method to estimate natural mortality. All axes are on a logarithmic scale. Species that are listed on CITES Appendix I or II are represented in blue, species listed on CMS Appendix I or II are represented as triangles. Species that are listed on neither CITES or CMS are indicated as grey circles.