Sublethal effects of alizarin complexone marking on Baltic cod (Gadus morhua) eggs and larvae

Standard, single-exposure alizarin complexone- (ALC) marking was conducted on early life stages of Baltic cod (Gadus morhua L.) to examine acute and chronic effects. Embryos and yolk sac larvae were marked using different concentrations of ALC (0, 50, 100 and 200 mg l− 1). Experiments included control groups for treatment and handling effects. In agreement with previous studies, long-lasting, distinct otolith marks were produced by immersion of yolk sac larvae in ≥ 50 mg l− 1 ALC for 24 h. Mortality of eggs and larvae was low during the marking procedure. Hatching success of ALC marked embryos was significantly reduced and hatching was delayed with increasing ALC concentration. Growth rates of larvae through 21 dph (exogenous feeding stage) were significantly reduced in ALC marked fish compared to controls. Biochemical condition (RNA–DNA ratio) was not affected. Subtle changes in activity and metabolism were indicated by reduced first feeding success and yolk absorption rates. Our results reveal that batch marking of finfish early life stages using ALC can have not only significant, acute impacts on survival but, despite long-standing assumptions to the contrary, also sublethal effects on other vital parameters like growth. We discuss these acute and chronic effects and give recommendations for assessment of life stage- and species-specific ALC immersion marking procedures.

► Baltic cod embryos and larvae were marked with ≥ 50 mg l− 1 Alizarin Complexone.
► Hatching success and survival of embryos was significantly reduced.
► Larval growth and survival was significantly reduced.
► Baltic cod was more susceptible than Atlantic cod and other species.
► Acute and chronic marking can affect stock enhancement program success.