Infestations of wild adult Atlantic salmon (Salmo salar L.) by the ectoparasitic copepod sea louse Lepeophtheirus salmonis Kroyer: prevalence, intensity and the spatial distribution of males and females on the host fish

The copepod Lepeophtheirus salmonis Kroyer is a specific ectoparasite of No rth Atlantic and Pacific salmonids in their marine phases. We compared infe stations of L. salmonis on wild Atlantic salmon (Salmo salar L.) captured i n estuarine (Firth of Tay, east Scotland; 1995, 1996) and marine coastal wa ters (Strathy Point, north Scotland; 1998, 1999). Host fish from the Tay we re caught by sweep netting, whilst those from Strathy Point were trapped in anchored bagnets. Fish capture method and exposure of the parasites to bra ckish conditions may both have detrimental effects on the retention of L. s almonis by the host, and hence possibly lead to their being under-estimated on returning adult fish. At Strathy Point, we recorded (i) an infestation prevalence of 100%, (ii) mean log abundances of pre-adult + adult L. salmon is at 19 (1998) and 24 (1999) per fish, (iii) 85/93% of all L. salmonis as being adults and (iv) overall 68/69% females. Fish caught in the upper Firt h of Tay showed significantly lower prevalences, intensities and abundances of L. salmonis and probably had lost part or all of their lice burdens pri or to capture, whereas those sampled from Strathy Point were apparently min imally affected by capture method or brackish water influences. The loss of parasites for the Tay fish was not markedly biased to males or females, or to pre-adult versus adult developmental stages. There were significantly g reater abundances of L. salmonis on two sea-winter fish (30 lice per fish) than on one sea-winter fish (17 lice per fish) sampled at Strathy Point in 1998. There are several possible explanations for such age-related patterns of abundance, but the indications are that (i) initial infestation of smol ts occurs in coastal waters, (ii) infestation of hosts in the open ocean is a persistent event, and (iii) oceanic reinfestation outweighs mortality lo sses of L. salmonis. This parasite typically occupies rather few zones on t he host fish covering only a small percentage of the total available body s urface area. Female predominance appears to be characteristic of L. salmoni s infestations of wild Atlantic salmon; this is in marked contrast to repor ts of extreme male dominance on farmed stocks. Adult females predominated o n the epidermis adjacent to, and posterior of, the insertion of the anal fi n and along the posterior dorsal midline between the dorsal and caudal fins . Males predominated on the sides of the head and along the dorsal midline between the head and the dorsal fin. Mate guarding/precopulatory pairs are formed between pre-adult II females and adult males. The significant correl ation between the distribution of pre-adult females and adult males may be indicative of pre-adult females actively seeking out adult males, but more likely is due to the (large) adult females competitively ousting all smalle r life stages (female and male) from those preferred zones. Given the relat ively low fecundity of adult females, and observations of 100% prevalence o f L. salmonis, the infective planktonic copepodid stage evidently is extrem ely efficient at locating and establishing upon its host fish in the pelagi c environment.