Fitting growth models to otolith increments to reveal time-varying growth

Identifying changes in fish growth is important for accurate scientific advice used for fisheries management, because environmental change is affecting fish growth and size-at-age is a critical component of contemporary stock assessment methods. Growth-increment biochronologies are time series of growth-increments derived from hard parts of marine organisms that may reveal dynamics of somatic fish growth. Here we use time series of otolith increments of two fish stocks to fit and compare a biologically-derived growth model and a generalized statistical model. Both models produced similar trajectories of annual growth trends, but the biologically-based one was more precise and predicted smaller inter-annual fluctuations than the statistical model. The biologically-based model strongly indicated covariance between anabolic and catabolic rate among individuals. Otolith size-at-age did not closely match fish length-at-age, and consequently the growth model could not accurately hindcast observed fish length-at-age. For these reasons, fitted growth dynamics from otolith biochronologies may best suited to identify growth rate fluctuations, to understand past drivers of growth dynamics, and improve ecological forecast in the face of rapid environmental change.

Impact of hatch date on early life growth and survival of Mueller’s pearlside (Maurolicus muelleri) larvae and life-history consequences

Growth and survival of Maurolicus muelleri larvae in Herdlefjorden, Norway, were investigated by daily otolith increment analysis. While high egg densities were generally observed throughout the spawning season, three cohorts each with a narrow window of hatching dates were identified. The first of these cohorts was characterized by low growth and poor morphometric condition and disappeared from the fjord during autumn. High-resolution drift modeling indicated that Herdlefjorden had a net export of larvae and negligible import in the period cohort 1 disappeared. Yet, the advective loss rate of larvae was not considered high enough to explain the near complete disappearance of the first cohort. An otolith-based growth chronology indicated that growth conditions in Herdlefjorden improved noticeably around mid-September and remained favorable the following month. The analysis of daily otolith increments could thus be used to document within-season variability in larval growth and survival. The low and variable survival due to short-term fluctuations in environmental conditions indicate that multiple batch spawning is an adequately evolved life-history strategy for marine planktivorous fish such as M. muelleri.

A novel method for the age estimation of Saddletail snapper (Lutjanus malabaricus) using Fourier Transform-near infrared (FT-NIR) spectroscopy

Near infrared (NIR) spectroscopy was investigated as a potential rapid method of estimating fish age from whole otoliths of Saddletail snapper (Lutjanus malabaricus). Whole otoliths from 209 Saddletail snapper were extracted and the NIR spectral characteristics were acquired over a spectral range of 800–2780 nm. Partial least-squares models (PLS) were developed from the diffuse reflectance spectra and reference-validated age estimates (based on traditional sectioned otolith increments) to predict age for independent otolith samples. Predictive models developed for a specific season and geographical location performed poorly against a different season and geographical location. However, overall PLS regression statistics for predicting a combined population incorporating both geographic location and season variables were: coefficient of determination (R2) = 0.94, root mean square error of prediction (RMSEP) = 1.54 for age estimation, indicating that Saddletail age could be predicted within 1.5 increment counts. This level of accuracy suggests the method warrants further development for Saddletail snapper and may have potential for other fish species. A rapid method of fish age estimation could have the potential to reduce greatly both costs of time and materials in the assessment and management of commercial fisheries.

Balancing the odds: the relationship between growth and energy storage in juvenile snapper (Chrysophrys auratus : Sparidae)

Larval and early juvenile fish living in temperate environments with limited resources face conflicting metabolic demands. To optimise their survival probability, fish must balance the need to out-grow predators by maximising their growth, while also accumulating sufficient energy reserves to sustain them through winter. We tested the hypothesis that resource allocation in Chrysophrys auratus changed from maximising growth to maximising energy storage over their first summer–autumn by measuring recent growth rates from otolith increments, and lipid, protein and carbohydrate concentrations in fish. Lipid concentrations showed the greatest change over the growing season, increasing three-fold between summer and mid-autumn. Growth rates showed the opposite trend, being significantly lower in mid-autumn than in summer. The relationships between growth rate and lipid or protein concentrations were consistent with a shift in resource allocation from growth to storage. During summer and early autumn, growth rates were independent of protein or lipid concentrations, but during mid-autumn there were significant positive correlations between the variables. Biochemical concentrations were independent of fish size, indicating that the accumulation of sufficient energy reserves for winter is determined by the timing of settlement and subsequent feeding conditions, which are likely to affect interannual recruitment variability of this important species.

Variation in the growth of larval and juvenile snapper, Chrysophrys auratus (Sparidae)

For many fish species, growth and mortality of larvae are closely coupled, with faster-growing larvae generally experiencing higher survivorship in the plankton, which may lead to higher recruitment. Using back-calculated growth trajectories derived from otolith increments we used the modified Fry model to estimate the growth rate of larvae and early juveniles of the commercially important sparid, Chrysophrys auratus, at four sites around northern New Zealand. Back-calculated growth rates were used to test the hypothesis that fish with a short pelagic larval duration (≤20 days) grew faster than did fish with a long pelagic larval duration (>24 days) during both the larval and juvenile periods. At three of the four sites, fish with a short larval duration grew significantly faster during the larval period, and these larvae generally continued to have a larger size-at-age as juveniles up to 70-day-old. Growth rates for both the larval and early juvenile period were also found to vary significantly among the four sites and were found to be unrelated to differences in water temperature. Localised variation in early growth of C. auratus among sites may be important in helping explain differences in their contribution to the recruitment to C. auratus populations.

Population estimation of bocaccio (Sebastes paucispinis) based on larval production

We estimate the stock size of bocaccio ( Sebastes paucispinis ) in the Southern California Bight based on the production of larvae. Area-weighted total length compositions of preflexion larvae in 2002 and 2003 were determined from ichthyoplankton survey data. Larval length-to-age transition matrices were estimated from data derived from the enumeration of daily otolith increments. In combination, these results were used to estimate daily rates of larval production and mortality during surveys conducted in those years. Daily rates of larval production were expanded to annual production rates using information developed from the long-term seasonal distribution of larvae in CalCOFI surveys. Total annual larval production was translated to total spawning biomass by calculation of the female population weight-specific fecundity derived from a length-based life table analysis of adult reproductive parameters. Our results indicate that there were 6953 – 10 656 metric tons of bocaccio biomass (males and females >16 cm fork length) in the Southern California Bight during the survey years, which agrees with biomass estimates from a traditional stock assessment. Unique and problematic aspects of the reproductive biology of Sebastes are discussed, including multiple spawning, weight-specific fecundity that depends on size, and bias in fecundity data gathered from vitellogenic ovaries.