Comparative demography of skates: life-history correlates of productivity and implications for management

Abstract

Age-structured demographic models were constructed based on empirical estimates of longevity and maturity for five deepwater Bering Sea skates to investigate how observed differences in life history parameters affect population growth rates. Monte Carlo simulations were used to incorporate parameter uncertainty. Estimated population growth rates ranged from 1.045 to 1.129 yr(-1) and were lower than those reported for other Alaskan skates and most chondrichthyans. Population growth rates of these and other high-latitude skates increased with relative reproductive lifespan, but displayed no significant relationship with body size or depth distribution, suggesting that assemblage shifts may be difficult to predict for data-poor taxa. Elasticity analyses indicated that juvenile and adult survival had greater per-unit effects on population growth rates than did egg-case survival or fecundity. Population growth rate was affected more by uncertainty in age at maturity than maximum age. The results of this study indicate that if skates are deemed to be a management concern, gear modifications or depth-specific effort controls may be effective.

Figure 1. Influence of age at maturity and longevity on finite population growth rates (λ).

Mean estimates were produced using probabilistic, age-structured matrix models based on empirical estimates of longevity and maturity for five Bering Sea skate species: Bathyraja lindbergi, B. maculata, B. minispinosa, B. taranetzi, and B. trachura. Variability in the estimated age at maturity (A) affected λ with greater per-unit magnitude than variation in longevity (B). Error bars indicate 95% confidence intervals.

Figure 2. Elasticity analysis.

Predicted means and 95% confidence intervals (range bounded by 2.5^th and 97.5^th percentiles) of elasticities for five Bering Sea skate species. Mean values were estimated from 5,000 Monte Carlo simulations assuming independent and identically distributed vital rates among age classes.

Figure 3. Length frequency distributions of skate bycatch in the eastern Bering Sea.

Data were obtained from observers (National Marine Fisheries Service, Alaska Fisheries Science Center) during commercial fishery activity in the Bering Sea and Aleutian Islands management area from 2004 to 2012. Estimated size at birth (solid line) and size at maturity (dashed line) are indicated for each species: Bathyraja lindbergi (A), B. maculata (B), B. minispinosa (C), B. taranetzi (D), and B. trachura (E).

Figure 4. Correlates of population growth rates in high latitude skates.

Mean finite rates of population growth (λ) predicted from probabilistic, age-structured matrix models in relation to the proportion of lifespan devoted to maturation [the ratio of age at maturity (α) to longevity (ω)] (A), maximum total length (TL_max; B), and midpoint of depth range (C). Note that higher values of α:ω, reported as a proportion, reflect a shorter reproductive lifespan relative to the total lifespan. Data are shown for each of the five species analyzed in this study, in comparison to the mean and variance from nine other high-latitude skates (Table 5). Pearson’s product-moment correlation coefficients and related P-values are shown for each relationship. Error bars indicate 95% confidence intervals (among-species).

Figure 5. Correlates of population growth rates in Alaskan skates.

Mean finite rates of population growth (λ) predicted from probabilistic, age-structured matrix models in relation to the proportion of lifespan devoted to maturation [the ratio of age at maturity (α) to longevity (ω)] (A), maximum total length (TL_max; B), and midpoint of depth range (C). Note that higher values of α:ω, reported as a proportion, reflect a shorter reproductive lifespan relative to the total lifespan. Data are shown for each of the five species analyzed in this study (filled circles), in comparison to the four other species of Alaskan skates for which data were available (open circles; Table 5). Pearson’s product-moment correlation coefficients and related p-values are shown for each relationship.