Cage colour and post-harvest K+ concentration affect skin colour of Australian snapper Pagrus auratus (Bloch & Schneider, 1801)

In an attempt to improve post-harvest skin colour in cultured Australian snapper Pagrus auratus , a two-factor experiment was carried out to investigate the effects of a short-term change in cage colour before harvest, followed by immersion in K + -enriched solutions of different concentrations. Snapper supplemented with 39 mg unesterified astaxanthin kg −1 for 50 days were transferred to black (for 1 day) or white cages (for 1 or 7 days) before euthanasia by immersing fish in seawater ice slurries supplemented with 0, 150, 300, 450 or 600 mmol L −1 K + for 1 h. Each treatment was replicated with five snapper (mean weight=838 g) held individually within 0.2 m 3 cages. L* , a* and b* skin colour values of all fish were measured after removal from K + solutions at 0, 3, 6, 12, 24 and 48 h. After immersion in K + solutions, fish were stored on ice. Both cage colour and K + concentration significantly affected post-harvest skin colour ( P 0.05), and there was no interaction between these factors at any of the measurement times ( P> 0.05). Conditioning dark-coloured snapper in white surroundings for 1 day was sufficient to significantly improve skin lightness ( L* ) after death. Although there was no difference between skin lightness values for fish held for either 1 or 7 days in white cages at measurement times up to 12 h, fish held in white cages for 7 days had significantly higher L* values (i.e. they were lighter) after 24 and 48 h of storage on ice than those held only in white cages for 1 day. K + treatment also affected (improved) skin lightness post harvest although not until 24 and 48 h after removal of fish from solutions. Before this time, K + treatment had no effect on skin lightness. Snapper killed by seawater ice slurry darkened (lower L* ) markedly during the first 3 h of storage in contrast with all K + treatments that prevented darkening. After 24 and 48 h of storage on ice, fish exposed to 450 and 600 mmol L −1 K + were significantly lighter than fish from seawater ice slurries. In addition, skin redness ( a* ) and yellowness ( b* ) were strongly dependent on K + concentration. The initial decline in response to K + was overcome by a return of a* and b* values with time, most likely instigated by a redispersal of erythrosomes in skin erythrophores. Fish killed with 0 mmol L −1 K + maintained the highest a* and b* values after death, but were associated with darker (lower L* ) skin colouration. It is concluded that a combination of conditioning snapper in white surroundings for 1 day before harvest, followed by immersion in seawater ice slurries supplemented with 300–450 mmol L −1 K + improves skin pigmentation after >24 h of storage on ice.