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Fishery Ecology, Diet & Zooplanton Program

Gadid Growth Studies at Little Port Walter Marine Station

Pacific cod (Gadus macrocephalus) and walleye pollock (Theragra chalcogramma) are two of the most important commercial species in Alaskan fisheries and ecologically important as prey to marine mammals, seabirds, and fish. Yet little is known about the growth of young-of-the-year Pacific cod and walleye pollock over time. Survey data provides information on a fish’s size at a distinct time period, but very little research has been done concerning the growth of individual fish over an extended period of time under controlled conditions.

In August 2009, researchers at Auke Bay Laboratories’ Little Port Walter Marine Station collected young-of-the-year Pacific cod and walleye pollock that had congregated in a marine fish trap used to capture returning adult Chinook salmon. A total of 446 Pacific cod and 24 walleye pollock were collected and subsequently divided into two saltwater net pens (2.1 × 2.1 × 2.4 m) The fish adjusted easily to pelletized fish food composed of 52% protein, 22% fat, 12% ash, 10% moisture, 1% fiber, and 1.7% phosphorous. Length and weight data were taken bimonthly on a subset of the populations. Length frequency data were taken on both populations in November 2009, June 2010, and August 2010. Otoliths were collected from mortalities and will be analyzed for age validation.

see caption
Figure 4. Log transformed length-weight data for Pacific cod (a) and walleye pollock (b) from September 2009 to August 2010.

In June 2010, 50 Pacific cod and 8 walleye pollock were tagged with passive integrated transponder (PIT) tags to begin tracking growth rates of individuals. At this time the walleye pollock were moved into a single net pen and the Pacific cod were split into three 2.1 × 2.1 × 2.4 m net-pens of varying fish density: 197, 143, and 50 individuals in each net. The different densities were designed to see if growth would be affected by crowding. Unfortunately, there was a high rate of mortality after the June data point, possibly the result of handling stress and warmer than normal saltwater temperatures. Because of the increased mortality, all Pacific cod were moved into a single 3.7 m3 net-pen positioned farther from shore in August 2010. All remaining fish in both populations were PIT-tagged in August when the two groups consisted of 216 Pacific cod and 17 walleye pollock.

Specific growth rates between June and August 2010 of PIT-tagged Pacific cod ranged from 0.31% body wt/d to 1.15% body wt/d. Average specific growth of tagged Pacific cod was 0.74, ± 0.17 % body wt/d. Pacific cod grew rapidly in the fall of 2009, and then growth decreased through the winter months and began to increase in early spring through summer (Table 1a). No data were collected in fall 2009 for walleye pollock; however, their growth virtually ceased in winter and then increased rapidly in early spring, more so than their feed rate (Table 1b). Log transformed length to weight data show a strong linear relationship for Pacific cod and walleye pollock in the first year of captivity (Figure 4a and 4b).

Young-of-the-year Pacific cod and walleye pollock exhibit different annual growth patterns. Although both species were fed identical rations, growth of walleye pollock virtually ceased during winter and increased rapidly in the spring; growth of Pacific cod slowed only slightly through the winter and increased gradually throughout the spring and summer. The growth rate of walleye pollock exceeded the feeding rate in the early spring months, indicating that walleye pollock can use energy stored from the previous summer and fall as a hedge for the following year’s growth. This may prove to be an important survival strategy in years when prey is limited.

Table 1. Average fork length, average wet weight, specific growth rates (% G=(ln(Wt2)-ln(Wt1)/dt)*100), and feed rate for Pacific cod (a) and walleye pollock (b) from the first year of captivity.
Sample date Number Average length (mm) Average weight (g) % Body wt/d Feed Rate (%)
9/4/2009 100 106.13 11.95     2.00
11/4/2009 95 147.61 31.78 1.58 1.40
1/8/2010 101 163.65 42.08 0.43 0.60
3/20/2010 110 176.65 52.25 0.30 0.50
6/4/2010 80 214.54 90.41 0.71 1.00
8/12/2010 216 247.17 141.60 0.64 2.00
Sample date Number Average length (mm) Average weight (g) % Body wt/d Feed Rate (%)
9/4/2009             2.00
11/4/2009 5 141.80 22.08     1.40
1/8/2010 6 145.33 23.22 0.08 0.60
3/20/2010 9 169.78 36.44 0.63 0.50
6/4/2010 23 203.76 66.46 0.78 1.00
8/12/2010 17 228.82 93.95 0.49 2.00


By Angela Feldmann


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