Sound production by Atlantic Cod during spawning

—Using captive groups of Atlantic cod Gadus morhua from two Northwest Atlantic populations (western Scotian Shelf and southern Gulf of St. Lawrence), we quantified the temporal patterns and behavioral contexts of sound production during the spawning season. We found that sound production occurs most frequently during the peak of the spawning period, particularly after the onset of darkness. The rate of sound production by males in the western Scotian Shelf group was 8.4 times greater on average than that of the southern Gulf of St. Lawrence group, and this corresponded to differences in the mass of the sound-producing musculature. Based on behavioral observations during daylight hours, we found that sounds were associated with both courtship behavior and agonistic displays. Although sounds were produced on only 14.5% of the occasions in which a male and female swam together in a ‘‘ventral mount,’’ limited data suggest that ventral mounts accompanied by sound were more likely to lead to spawning. While almost all sounds produced by Atlantic cod in our study matched the short ‘‘grunt’’ type previously documented for this species, we report evidence for another sound, described as a ‘‘hum,’’ which occurs during the ventral mount immediately before gamete release. We hypothesize that sound production is related to competition among males for access to females and may help synchronize gamete release, underscoring the potential importance of sound production to Atlantic cod spawning behavior. Many fishes produce sounds to communicate information to other individuals as part of their social behavior. One of the most common contexts of sound production is during reproduction (Hawkins 1993). Under these circumstances, sounds are typically produced by males, often during agonistic interactions that may ultimately reflect competition among males for access to females. For example, territorial male bicolor damselfish Stegastes partitus produce ‘‘pops’’ when approached by another male (Myrberg 1972), and highamplitude sounds are produced by male croaking gourami Trichopsis vittata during agonistic encounters (Ladich et al. 1992). In both cases, sounds are typically accompanied by agonistic behaviors such as chases, lateral displays, and circling, and may induce withdrawal of the opponent. Reproductive sounds may also serve to advertise the presence and reproductive readiness of males to females and may even arouse reproductive activity in the latter. During the spawning season, male oyster toadfish Opsanus tau occupy well-defined areas on the seabed and emit ‘‘boatwhistle’’ sounds that attract gravid females (Gray and Winn 1961). Similarly, male haddock Melanogrammus aeglefinus occupy territories during the spawning period and produce long trains of repeated ‘‘knocks’’ believed to be attractive to females, drawing them to male territories to engage in courtship and mating (Hawkins et al. 1967; Hawkins and Amorim 2000). When a female haddock approaches a male and as courtship proceeds, the rate of knock repetition increases so as to appear as a continuous ‘‘hum’’ in the courtship stages immediately prior to gamete release. The progressive increase in rate of sound production as males become increasingly aroused may indicate to females a readiness to mate and help synchronize the release of eggs and sperm (Hawkins and Amorim 2000). Sound production by males might also provide a basis for mate choice by females. For instance, female bicolor damselfish use courtship sounds of conspecific males to locate nest sites (Myrberg et al. 1986) and acoustically mediated individual recognition has been documented for this species (Myrberg and Riggio 1985), thus providing a basis for mate assessment. Atlantic cod Gadus morhua, a broadcast-spawning marine demersal fish found on both sides of the North Atlantic, reproduces in water depths ranging from tens to hundreds of meters (Brander 1994; Morgan et al. 1997; Smedbol and Wroblewski 1997). Like many fishes, cod are capable of producing and detecting sound (Hawkins 1993). Atlantic cod produce sound using three pairs of drumming muscles exterior to the swim bladder wall (Brawn 1961a; Rowe and Hutchings 2004). Typically, their sounds have been described as short ‘‘grunts’’ (about 200 ms) with peak sound amplitudes at frequencies ranging between about 50 and 500 Hz, and the acoustic repertoire is believed to * Corresponding author: rowes@mar.dfo-mpo.gc.ca 1 Present address: Fisheries and Oceans Canada, Bedford Institute of Oceanography, Post Office Box 1006, Darthmouth Nova Scotia B2Y 4A2, Canada. Received April 1, 2004; accepted November 18, 2005 Published online March 28, 2006 529 Transactions of the American Fisheries Society 135:529–538, 2006 Copyright by the American Fisheries Society 2006 DOI: 10.1577/T04-061.1 [Article]