Growth and condition of larval rockfish in a Patagonian fjord-type inlet: role of hydrographic conditions and food availability

Abstract

A field experiment was conducted for 4 consecutive days in December 2010 in Hornopirén inlet, a fjord from Chilean Patagonia, in order to determine the impact of marked environmental differences on fish larval distribution and growth (i.e., otolith microstructure) of the rockfish Sebastes oculatus. Two layers were noticeable: a surface one (0–10 m depth), characterized by warmer temperature and lower salinity, separated by a strong thermo- and halocline from the deep layer (>10 m depth), with lower temperature and saltier waters. As a prey field, nauplii showed similar abundances between strata, but calanoid copepodites were more abundant below the thermocline. Larval S. oculatus were more abundant in the deep stratum. Temperature was negatively correlated with calanoid copepodites as well as with S. oculatus abundances. Larval density was positively correlated with copepodites but not with nauplii abundance. S. oculatus larvae from the surface layer grew faster (0.137 ± 0.006 mm day⁻¹) than those collected below the thermocline (0.103 ± 0.012 mm day⁻¹). Otolith size (radius) as well as the recent otolith growth index was similar in individuals collected from both water parcels; however, linear mixed-effects models indicate that microincrement widths were slightly wider in older larvae occurring above the thermocline. Conclusions are that water temperature was more important than food availability for larval fish growth and recent hydrographic conditions are affecting the growth trajectories of larval rockfish in a Chilean fjord.

Appendix

Details and formalities of the linear mixed model framework presented for better understanding of the present study.

Formally, the linear mixed model of the present experiment is summarized as:

$$\mathop y\nolimits_{ij} = X_{ij} \beta + b_{i} + \varepsilon_{ij}$$

(1)

where y _ij is the observed response of the growth measured as an increment band width of the otolith of a particular individual fish for observation j (depth) on a different individual i; X _ij is the experimental field treatment (including two depth levels), consisting of columns representing factor contrasts and covariates. β represents the mean population coefficient of the larvae at different stages (ages) at different depth strata. b _i is a random variable representing the deviation from the population mean abundance for the ith day (age of each larval fish), and ɛ _ij is a random variable representing the deviation in abundance for observation of j on day i from the mean increment width for day i.

To complete the statistical model we specify the distribution of the random variables, b _i , i = 1,…, M (random variables, age in days in this study, 27 days for larval fish at 0–10 m depth and 25 days for larval fish from 10 to 45 m depth) and ɛ _ij , i = 1,…, M; j = 1, …, n _i (levels of the two main depth). We modeled both (random variables from the observations of each treatments) as independent, constant variance and with a mean of zero. The variances are denoted by σ ² _b for the b _i or “between-age or day” variability, and σ ² for the ɛ _ij , or “within-age or day” variability. This is summarized in (2).

$$b_{i} \sim N\left( {0, \sigma_{b}^{2} } \right),\quad \varepsilon_{ij} \sim N\left( {0, \sigma^{2} } \right).$$

(2)