Survival of Northern Atlantic Cod (Gadus morhua) Eggs and Larvae when Exposed to Ice and Low Temperature

Abstract

Freeze resistance of eggs and larvae of Atlantic cod (Gadus morhua) from the northern cod stock was investigated to determine whether ice contact could affect survival during the spring spawning season off Newfoundland. Egg and larval homogenates did not appear to contain antifreeze proteins (mean freezing points −0.78 and −0.88 °C, respectively). However, cod eggs did not freeze at −1.8 °C in icy aerated seawater, could be undercooled to −4.0 °C in ice, and froze at temperatures between −4.1 and −1 7 °C; freeze resistance depended on the integrity of the chorion. Larvae withstood undercooling to −1.8 °C, provided they were not brought into direct contact with ice crystals, if directly touched with ice, larvae froze at −1.36 °C (feeding stage) or −1.34 °C (yolk-sac), approximately 0.5 °C lower than would be expected from the freezing temperatures of their body fluids. The nature of their external epithelium and delayed development of sensitive gill structures below 0 °C may contribute to larval freeze resistance. Cod eggs and larvae are found in spring off Newfoundland and Labrador, when sea temperatures can be as low as −1.8 °C and ice cover extensive. While cod eggs are remarkably freeze resistant, such environmental conditions may cause freezing mortalities in larval cod.

Freezing resistance of antifreeze-deficient larval Antarctic fish

“…Although the integument most likely plays a critical role in the freezing resistance of larval fishes, the mechanism may be more involved than it acting as a simple physical barrier to ice propagation. This is demonstrated by the AFP-lacking yolk-sac larvae of G. morhua freezing at -1.34±0.01°C (Valerio et al, 1992a), while the AFP-deficient G. acuticeps larvae resisted freezing to -3.6±0.1°C when ice was applied to the integument under very similar laboratory conditions (Table·1).…”

“…For eggs of both polar (Atlantic cod, Gadus morhua and capelin, Mallotus villosus) and temperate fish species (plaice, Pleuronectes platessa and rainbow trout, Oncorhynchus mykiss) the chorion acts as a significant barrier to ice propagation, allowing the intact eggs, which are hypo-osmotic to seawater, to survive while in contact with ice at temperatures far below any they would be exposed to in nature (Aarset and Jørgensen, 1988;Davenport et al, 1979;Harvey and Ashwood-Smith, 1982;Valerio et al, 1992a). Fully developed P. antarcticum larvae are similarly protected while in the egg, surviving both cooling to below -9°C in a frozen seawater droplet (Table·2) and encasement of the egg in ice in freezing seawater.…”

“…For polar species, this attests to the need for AFPs, as seawater temperatures can often be lower than the colligative FP of teleost blood and ice is common in the surface waters of the polar seas. The freezing resistance of larval fish has been investigated previously in only two species: capelin Mallotus villosus (Davenport and Stene, 1986), and Atlantic cod Gadus morhua (Valerio et al, 1992a), neither of which possessed AFPs at the time of the experiments. Under laboratory conditions, M. villosus and G. morhua larvae freeze by -1.4°C in the presence of ice, which is ~0.5°C below the FPs of their body fluids.…”

“…Under laboratory conditions, M. villosus and G. morhua larvae freeze by -1.4°C in the presence of ice, which is ~0.5°C below the FPs of their body fluids. The larvae of both species risk exposure to ice and temperatures below their organismal FPs after hatching into the cold surface waters early in the Arctic spring, and Valerio et al (1992a) states that 'in unusually cold years…the risk of [G. morhua] larval mortality directly attributable to freezing may be significant.' Incidentally, the larvae of another teleost, Zoarces viviparus, the viviparous blenny that synthesizes a type III AFP, are well protected by AFPs at the time of parturition Ramløv, 2001, 2002).…”

Effects of polynyas on the hatching season, early growth and survival of polar cod Boreogadus saida in the Laptev Sea

“…Kara, Laptev and Beaufort Sea) the larvae must hatch early in winter or spring to achieve the minimum pre-winter size because of slow growth at low temperature and low food availability A different interpretation is that winter temperature dictates the hatching season of polar cod by limiting larval survival. The sub-zero temperature prevailing in ice-filled waters slows the swimming activity of Atlantic cod Gadus morhua larvae, which become completely inactive at -1.8°C (Valerio et al 1992). In the Northeast Water, the feeding of polar cod larvae decreased with decreasing temperature in the range + 4 to -1.8°C (Michaud et al 1996).…”