|Cod Fish Biology, Ecology and Population Dynamics|
Arctic Cod Population Dynamics
Ben Laurel, Louise Copeman, Ron Heintz, and JJ Vollenweider
Specific Growth Rates and Lipid Content
|Arctic Cod Population Dynamics Jen Marsh and Franz Mueter|
|Age and Growth through Otoliths Tom Helser|
Final Report to CIAP and BOEM:
Helser, T. E., Colman, J. R., Anderl, D. M., Kastelle, C. R. 2016. Growth dynamics of Saffron cod (Eleginus gracilis) and Arctic cod (Boreogadus saida) in the Northern Bering and Chukchi Seas. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 50 pp. – DRAFT REPORT
Saffron cod (Eleginus gracilis) and Arctic cod (Boreogadus saida) are two circumpolar gadids that serve as critically important species responsible for energy transfer in Arctic food webs of the northern Bering and Chukchi Seas. To understand the potential effects of sea ice loss and warming temperatures on these species’ basic life history, information such as growth is needed. Yet to date, limited effort has been dedicated to the study of their growth dynamics. Based on a large sample of otoliths collected in the first comprehensive ecosystem integrated survey in the northern Bering and Chukchi Seas, procedures were developed to reliably estimate age from otolith growth zones and were used to study the growth dynamics of saffron and Arctic cod. Annual growth zone assignment was validated using oxygen isotope signatures in otoliths and otolith morphology analyzed and compared between species. Saffron cod attained larger asymptotic sizes (L∞ = 363 mm) and achieved their maximum size at a faster rate (K = 0.378) than Arctic cod (L∞ = 209 mm; K = 0.312). For both species, regional differences in growth were found (p < 0.01). Saffron cod grew to a significantly larger size at age in the northern Bering Sea when compared to the Chukchi Sea, particularly at younger ages. Arctic cod grew to smaller asymptotic size but at faster rates in the more northerly central (L∞ = 197 mm; K = 0.324) and southern Chukchi Sea (L∞ = 221 mm; K = 0.297) when compared to the northern Bering Sea (L∞ = 266 mm; K = 0.171), suggesting a possible cline in growth rates with more northerly latitudes. Comparison of growth to two periods separated by 30 years indicate that both species exhibited a decline in maximum size accompanied by higher instantaneous growth rates in more recent years.
To assess age composition and growth dynamics of Arctic cod and Saffron cod in the eastern Chukchi Sea during 2012.
Reference collections for both species have been developed and consist of approximately 100 otoliths selected in a random and representative manner across the full range of all sizes available and was used to develop initial precision analysis and provide a photographic library for qualitative description of processing and ageing methods. Please visit Tom’s AFSC website for more information.
Examples of Arctic cod and saffron cod otoliths spanning observed ages from the 2012 Arctic Eis survey illustrate microstructure and readability. Arctic cod otoliths are considerably smaller than saffron cod otoliths which is one reason why these otoliths were processed using the cut & toast method vs. thin sectioning used for saffron cod otoliths. Between-reader precision was estimated to be approximately 79% (+/- 0 years) for Arctic cod which is typical for other Bering Sea gadids aged by AFSC (Figure 1). Between-reader precision for saffron cod was considerably higher at 93% (+/- 0 years) with little evidence of between reader relative bias (Figure 2).
Arctic cod is a short lived gadid (8 years old based on the maximum age in sample) with the predominance of the population age composition less than three years of age (Figure 3). The average age of the historic samples was over a full year older than compared to the contemporary 2012 Arctic Eis samples; 2.97 vs. 1.74 years. Age-0 fish were omitted for this comparison since historic collections did not use surface or mid-water trawls which are more likely to capture younger fish. Fitted growth curves (using the von Bertalanffy function) to Arctic cod showed little difference between historic and contemporary survey collections. Arctic cod growth is relatively slow in comparison to saffron cod which appear to have a similar life expectancy. The instantaneous growth rate (K) was estimated to be 0.22-0.24 with an asymptotic maximum size of 251 mm (Figure 3).
In comparison saffron cod’s asymptotic maximum size is 291-335 mm with growth rates nearly double that of Arctic cod (K=0.42-0.59) (Figure 4). Growth of saffron cod appears to be slightly higher in more recent surveys in comparison to historic samples, however this is still under investigation. Average age was also one year older from historically collected samples (2.26 years vs. 1.61 years) (Figure 4).
Age estimates range from age-0 to age-5 for both sexes with mean lengths comparable until ages 4 and 5. Size at age for females surpasses that of males at older ages, although for both sexes there is high degree of variability in length at age. Age compositions show a similar relative distribution between the sexes with approximately 80% of the sample occurring below age-3 (Figure 5). Fitted von Bertalanffy growth curves indicate that females have a larger asymptotic size but achieve that size at a slower instantaneous rate than males. We are still processing and ageing approximately 1700 Arctic cod otoliths from both the bottom and surface trawls so results regards population age compositions and growth curves are still preliminary.
We also conducted high resolution micro sampling of saffron cod otoliths for stable oxygen isotopes in an effort to validate our ageing criteria and interpretation of otolith microstructure (Figure 6). Efforts are still underway to sample the smaller Arctic cod otoliths. Figure 6 shows a series of oxygen 18 isotopes for a saffron cod otolith that we micro sampled using our computer-aided milling system and measured with mass spectrometer. The red arrows shown on the otolith picture correspond to the translucent growth zones (winter marks). This verifies that band counts are seasonal and correctly interpreted. The sequence of del18O (open white circles) and del13C (black) from core to otolith margin shows the strong seasonal variation in oxygen, the maxima of which represent cool water, winter growth (Right panels). The trend is down over the age of the fish indicating an ontogenetic migration or preference for warmer water as the animal grows older. Alternatively the downward trend could be a warming climate signal (although that would not account for the magnitude in change) or both climate and preference for habitats with generally warmer water. Interpretation of inverse relationship between otolith oxygen 18 and temperature is based on our work (right hand panels) with Pacific cod otoliths in the Bering Sea.
|Specific Growth Rates of Arctic and Saffron Cod Ben Laurel, Louise Copeman, Ron Heintz, JJ Vollenweider|
To assess the growth dynamics of Arctic Cod and Saffron Cod through garden-variety feeding experiments at different incubation temperatures.
To assess the lipid classes and storage potential found in Chukchi Sea cold-water gadids.
These efforts represent the first specific growth rate estimates for our Arctic and Saffron Cod species (Figure 1). Arctic Cod maximum growth is seen below 10 degrees, with maximum levels around 5-6. In contrast, Saffron Cod growth increases with temperature up to 16 degrees (possibly farther!). Arctic Cod seem to occupy a uniquely Arctic niche space in terms of growth rates as they have the highest rates among Gadids at the lowest, zero degree temperature while immediately giving up that advantage at temperatures near 3-4 degrees (Figure 2). Saffron Cod rates increased over the entire range of temperatures and will be tested again at even higher temperatures. This could be related to their general preference to coastal, estuarine habitats which can be very elevated in temperatures in the Arctic depending upon oceanographic currents, coastal topography (lagoons), and terrestial freshwater runoff.
Lipid component analysis (Figure 3) is another methodology used to determine interplay between fish condition, diet and feeding relationships, and energetic content of marine predators such as cod. In this study, we are looking at Alaskan coastal nursery habitats specifically to determine animal condition across spatial and temporal variables. The influence of terrestrial and marine resources is an important factor in determining fish condition. Use of these resources over the developmental phases (ontogengy) shifts depending upon habitat. Overall, if trends and relationships exist, the establishment of critical habitat for early growth in cods could be determined.
An interesting aspect of this work is changing growth rates across ontogenetic shifts in life style, feeding, and maximum size, which are being evaluated.