For hundreds of years, cod have supported fisheries throughout the North Atlantic. However, recent stock collapses have caused many fisheries to be reduced and others even closed. These collapses and the failure of many cod populations to recover despite protection from fishing have highlighted the need for more information about the ecology and evolution of the species, particularly in relation to spawning behaviour.
Despite being of theoretical interest and practical importance, very little is known about Atlantic cod reproductive behaviour. The only two studies reported to date suggest complex mating patterns, the occurrence of behavioural and acoustic displays by males, mate choice by females, and alternative reproductive strategies among males. However, there is no information on the selective causes and consequences of these behaviours, nor the structure of the mating system.
Our research employs a quantitative approach to understand causes and consequences of variation in the mating system of Atlantic cod at the individual and population levels. We are incorporating both detailed experimental studies in the laboratory and observations of cod in the wild.
As part of our research program, we have been maintaining groups of cod from two different populations under identical conditions in a large aquarium at Dalhousie University and recording their spawning behaviour. All of our fish are tagged so that we can recognize individuals and through daily observation, we hope to determine whether there are differences in mating behaviour between populations or among individuals within populations. In addition, we will assess which characteristics are related to mating and reproductive success. For example, the evidence to date seems to suggest that males which are the most talented vocalists might also be the most popular among females.
Sound production by males is believed to be important to successful mating in cod and we wanted to examine patterns of variation in the size of their sound-producing "drumming" muscles in more detail. To this end, we have been sampling cod from the Southwest Scotian Shelf each month to quantify seasonal and individual variation in body condition and drumming muscle mass. So far, we have found that drumming muscle mass tends to increase with fish size. Furthermore, when we control for fish size, spawning males have larger drumming muscles than non-spawning males. In addition, males have larger drumming muscles than females both during the spawning and non-spawning seasons. Interestingly, males that are healthiest in condition also seem to produce the biggest drumming muscles in preparation for spawning. These observations are consistent with the idea that sound production by males during the spawning season is important to reproduction and male drumming ability could convey reliable information to females about mate quality.
As part of our continuing research on Atlantic cod, we will study sound production in more detail by examining the types and characteristics of sounds produced, the behavioural contexts in which sounds occur, and temporal patterns of sound production.
Knowledge of the spawning behaviour of Atlantic cod will likely contribute to better understanding of population dynamics and improved ability to predict the effects of fishing on populations.