Factors affecting estimates of size at age and growth in grey triggerfish Balistes capriscus from the northern Gulf of Mexico

Abstract

Growth zones in dorsal spines of grey triggerfish Balistes capriscus from the northern Gulf of Mexico were utilized to estimate growth and examine factors that may affect estimates of size at age. Age was estimated from dorsal-spine sections of 4687 individuals sampled from U.S. waters during 2003-2013, including both fishery-independent (n = 1312) and fishery-dependent (n = 3375) samples. Ninety-six per cent (n = 4498) of these sections were deemed suitable for ageing; average per cent error between two independent readers was 10·8%. Fork length (L ) ranged from 65 to 697 mm and age estimates from 0 to 14 years. Both sex and sample source (fishery-independent v. recreational) significantly affected estimated size at age for 2-6 year-old fish. Data were pooled between sources to fit sex-specific von Bertalanffy growth functions. Results for the female model were L = 387 mm L , k = 0·52 year and t = 0·01 year, while for males L = 405 mm L , k = 0·55 year and t = 0·02 year. These results were significantly different between sexes and indicate clear sexual dimorphism. Thus, growth should be modelled separately by sex when examining population parameters or conducting stock assessment modelling. The positive bias in estimates of size at age computed for recreational v. fishery-independent samples also has clear implications for stock assessment as growth functions computed with fishery-dependent samples would tend to overestimate stock productivity.

Age and growth of grey triggerfish Balistes capriscus from trans‐Atlantic populations

“…The second factor that makes comparisons between the populations difficult is that samples were obtained using different gears; fish from the west were collected with a combination of traps and hook‐and‐line and fish from the east were collected with trawls. Several studies have noted the impacts of gear selectivity on estimates related to size, age and growth in triggerfishes (Allman et al ., 2018; Rivera Hernández et al ., 2019; Thomas, 2018). One way to reduce gear‐related biases of population estimates is to combine samples caught by multiple gears (Allman et al ., 2018; Wilson et al ., 2015).…”

“…Several studies have noted the impacts of gear selectivity on estimates related to size, age and growth in triggerfishes (Allman et al ., 2018; Rivera Hernández et al ., 2019; Thomas, 2018). One way to reduce gear‐related biases of population estimates is to combine samples caught by multiple gears (Allman et al ., 2018; Wilson et al ., 2015). In the current study, the authors purposefully included triggerfish samples in the west from both fisheries‐independent and ‐dependent collections, so that they could reduce impacts of gear selectivity because West Dependent samples were caught via hook‐and‐line and West Independent samples were caught with traps.…”

“…When examining the differences between the West Dependent and West Independent samples in this study, the authors did document typical gear‐related biases; mainly West Dependent B. capriscus samples were significantly larger and younger compared to West Independent samples. A study on age and growth of B. capriscus from GOM documented similar differences between fisheries‐dependent and ‐independent samples (Allman et al ., 2018). As stated earlier, East Atlantic triggerfish samples were caught via trawls, yet another sampling gear.…”

“…The species grey triggerfish Balistes capriscus Gmelin, 1789 supports fisheries in the eastern and western Atlantic Ocean (Aggrey‐Fynn, 2013; Johnson & Saloman, 1984; Kacem & Neifar, 2014; Kelly‐Stormer et al ., 2017), and its biology and ecology have been investigated throughout most of its range. In waters of the Gulf of Mexico (GOM) and southeastern coast of the United States (SEUS), B. capriscus is moderately long‐lived (maximum reported age of 15 years; Johnson & Saloman, 1984) and reaches a reported maximum size of 697 mm fork length (Allman et al ., 2018). Western Atlantic B. capriscus spend their early juvenile stage in the water column, associated with floating mats of sargassum (Casazza & Ross, 2008; Ingram, 2001; Wells & Rooker, 2004), and then settle into and remain as adults in benthic habitats characterized by hard‐bottom and a range of complex structure including rocky outcroppings and natural low‐to‐moderate profile reefs of corals and sponges across the SEUS and GOM shelf (Miller & Richards, 1980; Muhling et al ., 2014; Sedberry et al ., 2006).…”

“…Quantitative assessments of fishery stocks can benefit from documentation of growth rates and growth rate change over time, because it is an important compensatory response of fish populations to fishing pressure (Ali et al ., 2003; Coleman et al ., 2004; Lorenzen & Enberg, 2002). The determination of a fish population's age structure and growth parameters is dependent on accurate and precise estimates of age, and therefore the impacts of age estimate uncertainty or bias can ultimately impact evaluation of other population metrics including estimates of survival, mortality and biomass (Allman et al ., 2018; Anderson et al ., 2008; Gamboa‐Salazar et al ., 2020; Pardo et al ., 2013). B. capriscus has been aged using the first dorsal spine for over 30 years.…”