OBSERVATIONS ON EARLY LIFE STAGES OF ATLANTIC TOMCOD, 



MICROGADUS TOMCOD 



R. H. Peterson,^ P. H. Johansen,^ and J. L. Metcalfe' 



ABSTRACT 



In southern New Brunswick, tomcod spawn in streams from late December to mid-January. The 

 benthic eggs hatch and newly hatched larvae drift to sea in mid-March to mid-April at which time 

 ocean temperatures are beginning to increase. Larval migration to sea is probably aided by active 

 swimming of larvae to the surface to fill the swim bladder, which must be filled within 24 hours of 

 hatching. Photopositivity of the larvae may assist in guiding larvae to the surface. 



Water content and specific gravity of eggs reared in 0%o were 2.8 mg and 1.030. Eggs reared at 

 10-30%o had about 2.3 mg water per egg. Specific gravity of eggs incubated in 10%o was constant for 27 

 days (at 2°-4° C) at 1.038, then decreased to 1.033. This decrease is associated with water uptake of 

 0.5-0.6 mg per egg and elimination of salt. The specific gravity of eggs incubated in 20%« declined 

 linearly from 1.044 to 1.037, associated with accumulation of 0.2 mg of water and elimination of a 

 greater salt load. The specific gravity of eggs incubated at 30%o declined linearly from 1.049 to 1.045, 

 associated with 0.1 mg water uptake and apparently insufficient salt elimination. Water uptake and 

 salt excretion problems are minimized for eggs reared in freshwater, and under the experimental 

 conditions described here. Normal development could not occur in continued exposure to 30%o. In 

 natural spawning areas, the incubation medium is freshwater for most of the total cycle, with seawater 

 invading the area only at extreme high tide. The salinity tolerance of tomcod eggs is compared with 

 that of freshwater and marine fish eggs in general. 



Calculation of specific gravity of egg solids may prove a useful indirect way to investigate salt 

 regulation in fish eggs. 



The Atlantic tomcod, Microgadus tomcod (Wal- 

 baum), is an anadromous species of coastal 

 streams from Newfoundland to Virginia. Adults 

 ascend the lower reaches of southern New 

 Brunswick streams in December and January. 

 These spawning migrations form the basis for a 

 recreational ice fishery in some larger rivers. An 

 annual commercial catch of about 200 t is said to 

 be taken from inshore waters of the northwest 

 Atlantic (Scott and Grossman 1973). Local dip net 

 fishermen take numbers of spawners for both 

 human and animal consumption. 



Details of the life history of the early stages 

 (e.g., time of hatching, time of descent into saltwa- 

 ter) have been little studied. Leim (1924) observed 

 that eggs would hatch in freshwater or saline wa- 

 ter, but larvae would survive only in saline water. 

 Booth ( 1967) found sperm motility to be maximal 

 in low salinities, and that salinities of 0-15%o per- 

 mitted the highest percentages of eggs to develop 



'Fisheries and Environmental Sciences, Fisheries and Oceans 

 Canada, Biological Station, St. Andrews, NB, E0G2X0, Canada. 



^Fisheries and Environmental Sciences, Fisheries and Oceans 

 Canada, Biological Station, St. Andrews, New Brunswick; pre- 

 sent address: Department of Biology, Queen's University, 

 Kingston, ON K7L 3N6, Canada. 



Manuscript accepted September 1979. 

 FISHERY BULLETIN: VOL. 78, NO. 1, 1980. 



to the blastula stage. Howe (1971) described the 

 food habits and growth rates of young tomcod in 

 the Weweantic River estuary, Mass. 



The early stages of tomcod development have 

 not been studied extensively; therefore, field 

 studies were performed to obtain information on 

 spawning habitat, rates of egg development, and 

 timing of larval descent to saltwater. Tomcod eggs 

 are deposited in areas subject to variable 

 salinities, so the embryonic development and 

 water balance of tomcod eggs reared in several 

 salinities were also investigated to see how the 

 responses of this species compare with those of 

 freshwater and marine species. 



METHODS 



Field Studies 



The mouth and estuary of Frost Fish Creek 

 (frost fish is a local name for tomcod) were chosen 

 as a study area because the stream hosts a large 

 and regular spawning migration of tomcod which 

 is undisturbed except for some local dip net 

 fishing. It is a small stream (2-4 m wide) forming a 

 common estuary with the Digdeguash River in 



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