TURNOVER RATES AND DISCRIMINATION FACTORS OF NITROGEN AND CARBON STABLE ISOTOPES IN SEAHORSE Hippocampus reidi JUVENILES FOLLOWING A DIET SWITCH

Juveniles of H. reidi were reared for 46 days on two different diets (A11 and B6) based on copepods  and  Artemia nauplii (Fig ure 1). Changes in δ¹³C and δ¹⁵ N were studied by modelling the period comprising days 11 and 46 ( only Artemia nauplii offered). Isotopic data from juveniles were described applying two first-order one compartment models, as functions of growth ( model G, relative dry weight increase) or development progress (model D, days).

For diet A11  (84.5%  final  survival) both fitted models predicted that isotopic equilibrium (δY_eq) for δ¹³C and δ¹⁵N would be reached after 62.7-69.7 and 46.7-47.9 days, respectively (Table 1) . However, for diet B6  (49.1%  final  survival)  the isotopic equilibrium was reached much earlier (2 0.1 to 21.7  days for δ¹³ C and 26.1 to 28.3 days for δ¹⁵N). The range of discrimination factor (Δδ) estimates for diet A11 (2.73-3.81 ‰ for δ¹³C and 1.84-2.17 ‰ for δ¹⁵N) was higher than for diet B6 (0.85-0.93‰ for δ¹³C and 1.37-1.40‰ for δ¹⁵N). Besides this, juveniles from diet A11 grew faster.

Table 1. Parameter estimates for best-fit models for diets A11 and B6. Turnover rates in models G (G₅₀, G₉₅; x-fold increase in biomass) and D (D₅₀, D₉₅ ; Days). Δδ: Trophic discrimination factor (Δδ = δX_eq - δX_diet).

The study confirmed (i ) changes in turnover rates and discriminatio n factors depending of the diet, and  (ii)  survival improvement with longer periods of feeding on copepods.

This study was financed by project CGL2015- 68110-R (Spanish Ministry of Economy, Industry and Competitiveness).