CAN STABLE ISOTOPES INDICATE THE GEOGRAPHICAL ORIGINS OF SEA LICE?  

The salmon louse Lepeophtheirus salmonis is considered the primary parasitic disease of salmon aquaculture and affects populations of both wild and farmed fish. Despite the current understanding of its widespread presence and detrimental effects on host salmonids, its complete life history strategy, particularly regarding overwintering, remains uncertain. The current study investigated the overwintering strategies of sea lice in Cobscook Bay, ME through stable isotope analysis (SIA). Separate populations of lice were collected from farmed Atlantic salmon in the Bay and from returning wild salmon at the Milford Dam, ME to determine if the isotopic signatures of lice from different fish stocks were unique. Preliminary studies experimented with sample processing protocols in order to establish the most efficient method for SIA of sea lice. Further analysis of δ¹³C and δ¹⁵N stable isotopes was used to track animal movement and food web interactions, and thus the origins of associated sea louse populations in the bay.

Samples that were stored in seawater versus distilled water yielded significantly different δ¹³C and δ¹⁵N values. Similarly, lice that were acidified had significantly different isotopic signatures compared to those that were not acidified during sample preparation.

All sea lice samples on farmed salmon were identified as Lepeophtheirus salmonis, whereas all specimens on wild fish were Argulus foliaceus. Isotopic signatures are expected to differ between lice from farmed and wild fish.

A better understanding of experimental influences on stable isotope results will improve interpretation of future results. Distinct salmon populations appear to host lice with respective and different isotopic signatures. Therefore, through use of SIA as a quantitative tool, knowledge of where sea lice originate from and how they survive the cold season will enhance the current understanding of sea lice ecology under cold stress and its effects on wild and farmed fish.