Role of the midnight sun: comparative growth of pelagic juvenile cod (Gadus morhua) from the Arcto-Norwegian and a Nova Scotian stock
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ICES Journal of Marine Science, 53: 827–836. 1996
Role of the midnight sun: comparative growth of pelagic
juvenile cod (Gadus morhua) from the Arcto-Norwegian
and a Nova Scotian stock
Iain M. Suthers and Svein Sundby
Suthers, I. M. and Sundby, S. 1996. Role of the midnight sun: comparative growth of
pelagic juvenile cod (Gadus morhua) from the Arcto-Norwegian and a Nova Scotian
stock. – ICES Journal of Marine Science, 53: 827–836.
Size-at-age of pelagic juvenile cod from the north-east Atlantic off northern Norway
was approximately twice that of cod from the north-west Atlantic, off south-western
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Nova Scotia, Canada. Arcto-Norwegian cod (AN, 17–48 mm standard length, SL,
34–90 d post-hatch), were sampled in July 1988 with a capelin pelagic trawl, while
south-west Nova cod (NAFO region 4X, 7–32 mm SL and 32–105 d post-hatch) were
sampled in May–June 1985–1986 with a Tucker trawl. Growth over the previous 14 d,
back-calculated from otolith daily growth increments was 0.71 mm d "1 and 0.33 mm
d "1 for AN and 4X cod respectively. Within and between stocks, water temperature
and zooplankton biomass were significantly correlated with the 14 d index (linear
model, r2 =0.42), but an ANCOVA model comparing the AN and 4X regions was
highly significant (r2 =0.71), indicating additional factors. Gear selection was not
found to be responsible. While genetic factors could produce this result, there is 48%
more time during May–July for visual feeding north of the 71)N latitude off Norway
compared with 43)N off eastern Canada, using the light intensity threshold for larval
cod feeding. Our hypothesis of light-limited feeding opportunity is consistent with a
size- and temperature-dependent consumption model, and with aquaculture methods,
as well as the necessity for fast growth in the short northern summer for over-winter
survival.
? 1996 International Council for the Exploration of the Sea
Key words: pelagic juvenile cod, otolith microstructure, growth, age, stock, popu-
lation, light, photoperiod, temperature, prey, consumption, model.
Received 27 September 1995; accepted 22 April 1996.
I. M. Suthers and S. Sundby: Institute of Marine Research, P.O. Box 1870-Nordnes,
5024 Bergen, Norway. I. M. Suthers present address: School of Biological Science,
University of New South Wales, Sydney, NSW 2052, Australia.
Introduction Year-class strength of the Arcto-Norwegian (AN)
stock is largely determined during the pelagic juvenile
Atlantic cod (Gadus morhua) are distributed over a stage (Sundby et al., 1989). Processes affecting the
large latitudinal range from Georges Bank at 40)N to growth and survival of pelagic juveniles are therefore of
Spitsbergen Island at 80)N, and at least 14 cod stocks great interest. Water temperature was significantly cor-
are recognized over this range for management purposes related with recent otolith growth off northern Norway
(Garrod, 1988). These stocks show considerable vari- (Suthers and Sundby, 1993), and is an important factor
ability in growth, primarily driven by differences in in recruitment variation (Ellertsen et al., 1989). Off
average water temperature (Brander, 1994). Inter- south-western Nova Scotia (NAFO division 4X) zoo-
regional comparisons of different stocks are a powerful plankton prey abundance, particularly the >1050 ìm
way to determine which factors shape life-history fraction, was significantly correlated with recent otolith
processes (Powell and Phonlor, 1986; Conover, 1990, growth of pelagic juvenile cod, whereas the range in
1992; Brander, 1995). Hjort (1920) notes that intra- temperature over the study area was only slight (Suthers
specific comparisons between the Arcto-Norwegian et al., 1989).
and Canadian cod stocks may reveal fundamental Pelagic juvenile growth may not only depend on
oceanographic processes that would be otherwise temperature and food, but also on other environ-
undetected. mental and genetic factors. The AN stock is located
1054–3139/96/050827+10 $18.00/0 ? 1996 International Council for the Exploration of the Sea828 I. M. Suthers and S. Sundby
Table 1. Summary of study area and samples. Note that both surveys occur 6–10 weeks
after hatching. Growth refers to SL age "1. See text for details.
Norwegian Canadian
Sampling period July 1988 May–June 1985, ’86
No. stations 21 87
Latitude 65–72)N 42.5–44)N
Longitude 12–22)E 65–67)W
Region Tromsøflaket Browns Bk., SW Nova
Spawning March/April February/March
Hatching April/May March/April
Gear
Type Pelagic trawl Tucker trawl
Mouth opening 30 m2 2.5 m2
Mesh 5 mm 1.6 mm
Tow speed 2.5 knots 3.5 knots
No. of cod 159 489
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Range
Size 17–48 mm 7–32 mm
Age 34–90 d 32–105 d
Water temperature 6.3–9.8)C 3.3–8.2)C
Zooplankton biomass 29–78 mg m "3 1–172 mg m "3
Growth, age 40–80 d 0.31–0.74 mm d "1 0.17–0.37 mm d "1
approximately 30) of latitude north of the 4X stock, with where the lowest biomass of zooplankton occurred, due
concomitant effects on daylight length and length of in part to ctenophore predation (Suthers and Frank,
growing season. Comparisons of larval and pelagic 1990).
juvenile cod growth across the species’ large latitudinal
range have not been examined. The aim of this study
was to compare the length-at-age of pelagic juvenile Materials and methods
cod from the two stocks, and to compare the recent Timing of the Norwegian and Canadian cruises with
14-day otolith growth with the environmental variables respect to hatching was similar, corresponding to 6–10
recorded at capture. weeks after hatching (Table 1). Norwegian collections of
The majority of AN cod spawn in the Lofoten area of pelagic juvenile cod were made off the north coast of
north Norway, north of the Arctic Circle during March Norway during 1–26 July 1988, at 21 stations between
and April when the ambient water temperatures of the 64)N and 72)N using a pelagic capelin trawl (Bjørke
eggs are 1–4)C (Ellertsen et al., 1989). Larvae are et al., 1989; Godø et al., 1993; Suthers and Sundby,
transported north-eastwards in the Norwegian Coastal 1993, Table 1). This trawl produces abundance estimates
Current, and by June/July become temporarily entrained and length frequencies of young cod similar to that of
in clockwise gyres situated over large offshore banks, the a 10 m2 MOCNESS (3 mm mesh, Bjørke et al., 1989;
largest of which is Tromsøflaket between 70 and 72)N Potter et al., 1990). Temperature was recorded at 20 m
(Bjørke and Sundby, 1987). Drift into the cooler Barents depth. Zooplankton biomass data were obtained from
Sea was shown to reduce condition, size-at-age and the annual Russian June–July survey of the Norwegian
recent growth compared to those from more southern, Sea and the Barents Sea, along five inshore/offshore
warmer (albeit smaller) spawning locations (Suthers and transects which overlapped 12 of the Norwegian
Sundby, 1993). stations. The usual survey area is shown in Degtereva
The spring spawning by 4X cod occurs a month et al. (1986), while the 1988 June–July zooplankton data
earlier in February and March on Browns Bank with was taken from the 1990 internal report of PINRO
water temperatures between 2–3)C. Larvae and pelagic (V. N. Nesterova, 6 Knipovich St, Murmansk 183763,
juveniles tend to be retained in a clockwise gyre over Russia). A vertical haul with a 37 cm diameter Juday net
Browns Bank (43)N), and are episodically released and (250 ìm mesh) from 50 m to the surface was made at
advected northerly in the residual current (Campana each station. The average zooplankton concentration
et al., 1989; Smith, 1989; Suthers et al., 1989). Release over three broad regions covering 17 of the 21 stations
and drift of pelagic juveniles from the Browns Bank gyre was determined by integrating with a planimeter. The
into the nearshore zone (Campana et al., 1989) may wet-weight of zooplankton (mg m "3) was converted to
negatively impact growth and survival in this stock, dry weight by multiplying by 0.19 (Wiborg, 1960).Growth of Norwegian and Canadian pelagic juvenile cod 829
Eastern Canadian pelagic juvenile cod were sampled 50
on Browns Bank and off south-western Nova Scotia
during three cruises during 16–22 May 1985, 10–23 May
1986, and 5–11 June 1986, at 87 stations between 42.5)N 40
and 44)N, using a Tucker trawl (Suthers and Frank,
1989, Table 1). Temperature was recorded using a
Standard length
temperature/depth meter at 20 m depth (or at the surface 30
when the depth was less than 30 m). Zooplankton bio-
mass was collected either consecutively (with a 0.25 m2
BIONESS net, 333 ìm mesh) or simultaneously (with a 20
0.5 m diameter ring net, 405 ìm mesh), and dried to a
constant weight in the laboratory.
10
Analysis
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0
The lapillar otoliths were extracted from 648 fish 20 40 60 80 100 120
(Table 1) and prepared as described in Campana (1987, Age
1989), and results of known-age material are provided in Figure 1. Scatter plot of size on age for Canadian (4X, +) and
Suthers and Sundby (1993). The radius of the lapillus Norwegian (AN, ,) pelagic juvenile cod (see Equations 4
from hatch-check to periphery is linearly related to and 5).
standard length (SL), and slopes and intercepts were
significantly different between 4X and AN fish
(ANCOVA, p14 mm transferred from 5% formalin to 95% growth model and appears typical for larval cod off
ethanol shrink830 I. M. Suthers and S. Sundby
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Figure 2. Photograph of a lapillus from similar sized pelagic juvenile cod, (a) AN cod, 24.2 mm, 65 d post-hatch, from
Trømsoflaket, July 1988 and (b) 4X cod, 26.0 mm, 89 d post-hatch, from off Yarmouth, Nova Scotia, June 1986. Radius
from hatch check to edge is shown in black, with the first 15 daily growth increments indicated. Scale bar=32 ìm.
The range in published growth rates for wild-caught possibly be an artefact, as larger or more efficient gear
cod larvae, using a variety of gear types shows a types may have selected larger and faster growing larvae.
doubling between geographic regions (Fig. 3, Table 2), This possibility is explored below.
with AN pelagic juveniles having the greatest size-at-age.
Georges Bank cod exhibit a considerable range in size
Size differences and gear selection
at age between studies (Boltz and Lough, 1983, 1988;
Campana and Hurley, 1989), but Campana and Hurley The larger Norwegian pelagic trawl (30 m2 effective
(1989) ascribed this difference to shrinkage correction in mouth opening) captured cod which were about 50%
the earlier study, by comparing lapillar radius growth. larger than those from the Canadian Tucker trawl
However, the difference between AN and 4X cod could (2.5 m2 mouth opening, Table 1, Fig. 4). Using twoGrowth of Norwegian and Canadian pelagic juvenile cod 831
80 40
a
Est. standard length (mm)
60 b 30
Frequency (%)
AN
40
20
4X
c
20
10
d, e
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0 25 50 75 100 125
Age (d) 0
5 10 15 20 25 30 35 40 45 50
Figure 3. Average relationship of standard length on age for Standard length
larvae from a variety of geographic regions as listed in Table 2;
AN, Arcto-Norwegian cod; 4X, Nova Scotian cod; a, reared Figure 4. Length–frequency histogram for the two studies. Size
cod at 70)N (Olsen et al., 1991); b, reared cod at 60)N (Blom interval at 5 mm, for example, includes cod 5–10 mm (. 4X,
et al., 1994); c, Georges Bank and shrinkage corrected (Bolz / AN).
and Lough, 1988); d, e, Browns Bank, Georges Bank (Campana
and Hurley, 1989).
Role of temperature and prey abundance
other similar gear-types, Munk (1993) compared the The most significant external factor influencing fish
length frequency of pelagic juvenile cod sampled by a growth is water temperature (Campana and Hurley,
2 m diameter ring net (3 m2, and 1.6 mm mesh), and an 1989; Brander, 1994), followed by prey abundance
IYGPT trawl (100 m2, with 5 mm mesh cod end). Size (Ellertsen et al., 1980; Bailey, 1989; Suthers et al., 1989).
selection of cod by the 2 m ring net was obvious, as the Campana and Hurley (1989) calculated the vernal
abundance of cod >36 mm abruptly declined to about change in temperature from a parabolic function, fitted
25% of that sampled by the IYGPT. However for the to observed temperatures, and incorporated these into a
Canadian Tucker trawl, 99% of 4X cod were832 I. M. Suthers and S. Sundby
Table 3. Regression models of increase in standard length over 14 d prior to capture (äSL14) as the
dependent variable, incorporating age (AGE), water temperature (TEMP), ln[zooplankton] biomass
(LZ), and REGION (or stock, 4X/AN). Region is a categorical variable with two levels (4X=1,
AN= "1). See Table 1 for range in the independent variables. All variables listed are highly significant
(pGrowth of Norwegian and Canadian pelagic juvenile cod 833
70°N 125
24
100
20
Daylength (h)
Consumption
75
16 45°N
50
12
25
March April May June July August
8 0
60 90 120 150 180 210 240 5 10 15 20 25 30 35
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Day-of-year Dry weight
Figure 6. Daylength in hours, represented as the time from civil Figure 7. Results of the size and temperature-dependent con-
twilight sunrise to civil twilight sunset at 45)N and 70)N for the sumption model in J d "1 of Blom et al. (1991) for Norwegian
period from March through August. and Canadian cod in the common size range of 18–28 mm SL.
Dry weights are estimated from standard length (see Methods)
(+ 4X, , AN).
Variation in roughness of the sea surface, induced by
wind, will considerably influence the reflectivity, and
hence the light conditions in general when the sun is higher than for 4X cod between 10–40 mg dry weight.834 I. M. Suthers and S. Sundby
Kendall and Nakatani (1991) compared the early life winter, regardless of the effect of daylength. There are
history of the walleye pollock (Theragra chalcogramma) few comparative studies of AN cod with other stocks,
from the eastern and western sides of the north Pacific. and the results are equivocal (Gamble and Houde, 1984;
At 50 d post-hatch, larvae from Funka Bay, Japan van der Meeren et al., 1994). Alternatively, the quality of
(42–46)N) were 14.0 mm SL, whereas those from light varies with latitude. The spectral distribution is
Shelikof Strait, Alaska (56–59)N) were 14.8–18.7 mm removed to higher wavelengths as the solar altitude
SL. Temperature at spawning is 2–6)C in Japan and decreases. Nevertheless, the difference in pelagic juvenile
5.5)C in Alaska (Kendall and Nakatani, 1991), and size disappears over the long Norwegian winter, as AN
would presumably be more similar during spring cod at ages 1 and 2 average 14 and 23 cm (Anon., 1990),
growth. This 2–3 mm difference is consistent with while those from Browns Bank average 21 and 33 cm
approximately 2 h difference in daylength during the (Campana and Hamel, 1992).
spring occurrence of larvae, although the difference in This study found a large difference in growth rate of
naupliar prey concentration and natural mortality rates pelagic juvenile cod between 43–70)N, which is not due
are uncertain (Kendall and Nakatani, 1991). to gear selection. The key environmental difference in
Capelin (Mallotus villosus) co-occur with cod on both this study is argued to be daylength. Fish larvae are
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sides of the Atlantic. If light level is affecting feeding visual predators, and there is a variety of theoretical,
opportunity, it should also affect other species similarly. laboratory and field evidence that larval feeding is
The mean size of Barents Sea capelin larvae in early June light-limited. The availability of prey is not simply a
1981–1984 was 10.9–13.5 mm (Alvheim, 1984), and by function of prey biomass, but includes light and the
late August of those years, pelagic juvenile capelin were effect of turbulence (Sundby et al., 1994). We suggest
40–53 mm (Loeng and Gjøsæter, 1990). The overall that 6–8 h additional daylight per day can almost double
growth rate was therefore 0.36–0.50 mm d "1 at tem- the size of pelagic juvenile cod by mid-summer. The
peratures of 5.3–6)C. In comparison, growth rates of almost 50% increase in feeding time results in only
larval capelin in the Gulf of St Lawrence averaged 33–40% increase in modelled consumption and actual
0.25 mm d "1 in 1974–1975 (2–14)C, 5–30 mm size growth, because of the effect of cloud cover, and
range, Jacquaz et al., 1977), approximately 50% slower possible physiological limits.
than Barents Sea capelin. While prey, temperature and The implication of our interpretation is that feeding
non-linear growth may confound this comparison, the by larval cod may be influenced by cloud cover, particu-
general trend is consistent with light-limited growth. larly at dawn and dusk. Persistent cloud cover, fog,
phytoplankton blooms or turbidity could result in poor
feeding and growth. It would be of interest to determine
Genetic differences and other factors
which stages of cod development benefit the most from
In our hypothesis of light-limited growth in the ocean, continuous light. Our interpretation underscores the
we are assuming that many other potentially important importance of light availability, and the quality of light
factors may be equal between regions, or could not for larval fish feeding in the sea generally. Many tropical
produce such a large effect. A similar latitudinal gradient ocean fish larvae occur near the thermocline/nutricline,
in growth (32–45)N) has been observed in Atlantic at depths of 60–100 m depth (Loeb, 1979), where light
silversides (Menidia menidia) along the east coast of levels are about 1% of surface light. Feeding may be
North America (Conover and Present, 1990). Despite light-limited here as well.
growing seasons being more than twice as long at 32)N,
juveniles at 45)N are at the same size by the summer’s
end. This growth difference has a genetic basis (Conover Acknowledgements
and Present, 1990). Doubling in growth rates is also The financial support of the Norwegian Research
observed in other species of juvenile fish between Council for Natural Sciences is greatly appreciated. Per
28–46)N, and is anecdotally observed in invertebrates Solemdal brought to our attention Hjort’s observation
(Conover, 1990). Larger fish tend to survive their first on trans-Atlantic comparisons. Herman Bjørke
winter better than smaller fish (Post and Evans, 1989), provided considerable assistance with the database
indicating a probable selective cause. Demersal juvenile and uncovering references. Geir Blom alerted us to
cod are cannibalistic in years of low capelin abundance, Conover’s work. Drs S. E. Campana and K. Brander
and the effect of size selective mortality on 0-group cod critically reviewed the manuscript.
in the Barents Sea may be considerable. For example a
significant proportion of the 1984 and 1985 year class of
AN cod was eaten by the 1983 year class (Sundby and References
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