Mate limitation in sea lice infesting wild salmon hosts: the influence of parasite sex ratio and aggregation

Abstract

Mate limitation in dioecious parasite species has the potential to impact parasite population growth. Our focus of interest was the influence of parasite sex distribution among hosts on parasite reproduction and transmission dynamics for populations of ectoparasitic sea lice (Lepeophtheirus salmonis Krøyer) establishing on wild juvenile salmon hosts. The data included more than 139,000 out‐migrating juvenile pink salmon (Oncorhynchus gorbuscha (Walbaum)) and chum salmon (Oncorhynchus keta (Walbaum)) in British Columbia, Canada, sampled over nine years. For almost all years, the sex ratio of the reproductive stages of the sea lice was female‐biased. The probability of a female being able to mate (i.e., of being attached to a fish also carrying a male louse) increased with increasing parasite abundance and parasite aggregation. We compared, with expected modeling predictions, the observed prevalence of pairs of sea lice (i.e., one reproductive louse of each sex) on a given fish and the observed probability of a female being able to mate. These comparisons showed that male and female sea lice tend to be distributed together rather than separately on hosts. Distribution together means that sea lice are distributed randomly on hosts according to a common negative binomial distribution, whereas distribution separately means that males are distributed according to a negative binomial and females are distributed in their own negative binomial among hosts. Despite the tendency for distribution together we found that, in every year, at least 30% of reproductive female sea lice experience mate limitation. This Allee effect will result in submaximal rates of parasite reproduction at low parasite abundances and may limit parasite transmission. The work has important implications for salmon parasite management and the health both of captive farm salmon populations and migratory wild stocks. More broadly, these results demonstrate the potential impact of mate limitation as a constraint to the establishment and spread of wild ectoparasite populations.

Sex differences in the early life stages of the salmon louse Lepeophtheirus salmonis (Copepoda: Caligidae)

“…Norwegian law demands that salmon louse numbers are collected in aquaculture regularly [ 40 ]; however, these numbers cannot be used to deduce the sex ratio of the population. Adult female lice are counted separately, while adult males, preadult males, and preadult females are collectively classified as “mobile stages.” [ 41 ] analyzed six studies reporting male and female counts from wild and farmed Atlantic salmon and they could not find a generally valid sex ratio. In contrast, they concluded that host population, parasite population, and environment might influence the sex ratio of salmon lice [ 41 ].…”

“…Adult female lice are counted separately, while adult males, preadult males, and preadult females are collectively classified as “mobile stages.” [ 41 ] analyzed six studies reporting male and female counts from wild and farmed Atlantic salmon and they could not find a generally valid sex ratio. In contrast, they concluded that host population, parasite population, and environment might influence the sex ratio of salmon lice [ 41 ]. Nevertheless, there seems to be a tendency to female-biased populations in the sea and male-biased populations in aquaculture [ 38 ].…”

Sterilization of sea lice eggs with ultraviolet C light: towards a new preventative technique for aquaculture

“…Sterilized individuals, especially males, can disproportionately impact populations by deceiving conspecifics into investing in infertile matings . This strategy has been successfully used to control other pest species, including the release of male mosquitos sterilized by radiation to function as ‘ovitraps’ for female mosquitos, and may be particularly effective against sea lice as female lice are frequently mate‐limited …”

“…30 This strategy has been successfully used to control other pest species, including the release of male mosquitos sterilized by radiation to function as 'ovitraps' for female mosquitos, 31 and may be particularly effective against sea lice as female lice are frequently mate-limited. 32 Lethal or sterilizing doses may conceivably be applied to all lice life stages. Combined with existing lice control methods to minimize encounter rates or remove attached lice from stock, UVC has potential to assist in the ongoing suppression of sea lice populations.…”

Modelling sea lice control by lumpfish on Atlantic salmon farms: interactions with mate limitation, temperature and treatment rules

“…This situation reflects an assumption made in many sea louse population models (Revie et al 2005, Rittenhouse et al 2016. The option allowed us to explore the influence of lumpfish and chemical controls in conjunction with mate limitation (Krkošek et al 2012, Stormoen et al 2013, Groner et al 2014, Cox et al 2017.…”