Growth and maturation of rainbow smelt (Osmerus mordax) at the northern limit of their distribution range (Lake Melville, Labrador): Support for the hypothesized temperature-size rule

Rainbow smelt (Osmerus mordax), a small pelagic fish found throughout the northwest Atlantic Ocean, experienced a northward retreat of its distribution range over the 20th century and an overall decline in biomass in recent decades, both attributed to warming waters and habitat loss. In the Lake Melville estuary (Labrador, Canada), at the northern limit of its distribution range, smelt represents the main forage species and faces environmental changes from both climate warming and hydroelectric dams. This study aims to improve our understanding of the ecology of rainbow smelt and its vulnerability to climate and anthropogenic changes. We investigated hatching period, growth, and maturation of rainbow smelt in the Lake Melville estuary using trawl surveys and otolithometry. Growth and maturity from rainbow smelt in Lake Melville were then compared to southern populations to test the temperature-size rule, which stipulates slower growth and larger length-at-age of maturity at colder temperatures. In accordance with this rule, adult rainbow smelt in Lake Melville grew slower, matured up to 2 years later, and lived up to 3 years longer compared to southern populations. In contrast to it, larval growth rate was up to 3 times faster than values reported for a more southern population. Our results demonstrate that rainbow smelt in Lake Melville are well adapted to the short growth season and cold water temperatures prevailing at sub-arctic latitudes. Yet, if variation in life history across their range is a plastic response to thermal gradients, populations at their northern limits could benefit from a longer and warmer growth season.