Journal of Fish Biology (2014) doi:10.1111/jfb.12342, available online at wileyonlinelibrary.com Temporal variation and characterization of grunt sounds produced by Atlantic cod Gadus morhua and pollack Pollachius pollachius during the spawning season L. J. Wilson* †‡, M. T. Burrows*, G. D. Hastie† and B. Wilson* *Scottish Association for Marine Science, Scottish Marine Institute, Oban, Argyll PA37 1QA, U.K. and †Sea Mammal Research Unit, Scottish Oceans Institute, University of St Andrews, St Andrews, Fife KY16 8LB, U.K. (Received 4 April 2013, Accepted 13 January 2014) Fine-scale temporal patterning in grunt production and variation in grunt attributes in Atlantic cod Gadus morhua and pollack Pollachius pollachius was examined. Pollachius pollachius produced only a single sound type, the grunt, similar to that previously described for G. morhua. Sound production and egg production were correlated in P. pollachius but not in G. morhua. Only G. morhua displayed a strongly cyclical pattern, producing more grunts at night. Finer-scale temporal patterning in grunt production was observed in both species which produced signiicantly fewer grunts following a period of high grunt production. These quieter periods lasted up to 45 min for P. pollachius and up to 1 h in G. morhua. Grunts were not always produced in isolation but organized into bouts in both species. Longer bouts were more frequent during periods of increased sound activity and were linked with changes in grunt characteristics including increased grunt duration, pulse duration and repetition period of each pulse combined with decreased dominant frequency. This study provides the irst evidence of acoustic signalling being used by spawning P. pollachius and presents the most detailed analysis of the complexity of gadoid sound production. © 2014 The Fisheries Society of the British Isles Key words: acoustic communication; acoustic signal; gadoid; mate choice; sound production. INTRODUCTION Acoustic signals are used by many animals to convey true messages in which both sender and receiver beneit from the information exchange. Message attributes may show variability in features depending on the type of message; characteristics such as signal recurrence rate or intensity may indicate motivation or condition, provid- ing important information for mate choice and male–male interactions (Bradbury & Vehrencamp, 1998; Rowe, 1999). The mating decisions that females make, particu- larly about their choice of mate, inluence individual and offspring itness, and as such female mating decisions are under strong evolutionary pressure (Barbosa & Magurran, 2006). Sound production in ishes is commonly detected in a reproductive context and attributed to mate attraction (Gray & Winn, 1961; Amorim et al., 2003), the exclusion ‡Author to whom correspondence should be addressed. Tel.: +44 1334 463446; email: ljw33@st-andrews.ac.uk 1 © 2014 The Fisheries Society of the British Isles