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

• Published in: Ecosphere
• Main Authors: Cox, Ruth, Groner, Maya, Todd, Christopher David, Gettinby, George, Patanasatienkul, Poo, Revie, Crawford
• Other Authors: University of St Andrews. School of Biology, University of St Andrews. Marine Alliance for Science & Technology Scotland, University of St Andrews. Scottish Oceans Institute
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: Mate limitation; Sex ratio; Host-parasite interaction; Dioecious parasite; Sea lice; Wild salmon; QH301 Biology; QL Zoology; 3rd-DAS; QH301; QL; British Columbia; Canada; Keta; Oncorhynchus gorbuscha; Pink salmon
• Online Access: http://hdl.handle.net/10023/12412; https://doi.org/10.1002/ecs2.2040

Research was undertaken thanks to funding from the SALMODIS project and the Canada Excellence Research Chairs Program. In particular, the CERC Aquatic Epidemiology Visiting Scientist initiative provided support for an extended visit by C.D.T. to work with colleagues at UPEI. 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 amongst 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 (O. 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 modelling 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 30 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 sub-maximal rates of parasite reproduction at low parasite abundances and may limit parasite transmission. ...