DFO Contract Report
A Determination of the Ecosystem Carrying
Capacity for Finfish in Gilbert Bay Labrador:
A Marine Protected Area
Dr. Joe Wroblewski
Ocean Science Centre
Memorial University of Newfoundland
7 April 2006
The views expressed in this document are solely those of the respective authors, and
should not be characterized as reflecting the views or policies of the Department of
Fisheries and Oceans nor the Government of Canada.
INTRODUCTION
Gilbert Bay (Figure 1) is a narrow inlet that located on the southeast coast of
Labrador. With 25 km long and 1-3 km wide, it covers a total area of approximately 60
km2. On October 11, 2005 Gilbert Bay was announced as a new marine protected area
(MPA). While preserves the unique habitat to a wide range of marine species, Gilbert
Bay MPA was designed mainly to protect the genetically distinct resident species of
northern cod, which is found only in this area (Green and Wroblewski, 2000; Ruzzante et
al., 2000; Beachem et al., 2002; Morris and Green, 2002). Based on the regulation, no
fishing for Atlantic cod within the MPA is allowed, except a recreational/food fishery for
cod in Zone 3 (Figure 2). In order to provide basic information to the Gilbert Bay
Steering Committee on their further recommendation on cod fishery management, in
August 2005, we proposed to do the first-order calculation of carrying capacity for finfish
in Gilbert Bay.
This study is based on the bottom-up mechanise to estimate the probable fish
production in Gilbert Bay by examining the transferred from primary production to fish
production. Gilbert Bay is a suitable home to a wide range of marine species, which
include marine plants, fish, and marine mammals. In the survey of fish fauna during
2004, 15 families and 25 species were collected from Gilbert Bay (Wroblewski et al.,
2006). Among these species, some of them present in the bay all year-round are
considered as resident species, such as, Atlantic cod, Greenland cod, sculpin, limpfish,
and winter flounder, the others enter the bay for spawning, feeding, or transiting
(Wroblewski et al., 2006). We also partitioned the theoretical total fish production
according to the information from 2004 standard survey study.
METHODS
Ecosystem carrying capacity refers to the maximum fish biomass that an
ecosystem can sustain. The bottom-up approach has been widely used in estimating
annual fish production based on the primary production (Ryther, 1969; Ware, 2000;
Harrison and Parsons, 2000; Ware and Thompson, 2005). Mills and Fournier (1979)
suggest that the structure of the ecosystems must be taken into account. In this study, we
followed Harrison and Parsons (2000) by using the primary production and trophic level
to estimate annual fish production and ecosystem carrying capacity of Atlantic cod in the
bay. A relationship between the total annual primary production (PP, t km-2 year-1) and
annual fish production (FP, t km-2 year-1) is given as follow (Harrison and Parsons, 2000):
FP = Rf ML PP TE (TL-1)
where Rf is the fraction of PP retained within the system, ML is the fraction of PP and
heterotrophic microzooplankton production derived from the microbial loop that is
potentially available to be consumed by mesoplankton, TE is transfer efficiency, and TL
is fractional trophic level of fish species, where net phytoplankton represent the first
trophic level, which TL=1 (Pauly, 1998). Also described by Harrison and Parsons (2000),
the ratio of annual fish production (FP, t km-2 year-1) to biomass (B, t km-2) is a function
of its average body weight in gram (W):
FP / B = q W-0.25
where q is species-specific parameter (2.2 for Atlantic cod).
Harrison and Parsons (2000) calculated the fraction of the total primary
production available to the mesozooplankton, Rf *ML, is 40% in North Sea. The transfer
efficiency (ET) is various among different ecosystem. While Slobodkin (1961) thinks that
it may be 10%, Schaeffer (1965) suggests that 20% is possible. We used 15% as Ryther
(1969) assign to the coastal province. Gilbert Bay belongs to subarctic zone. With the
effect of cold Labrador Current, the waters in Gilbert Bay may remain at subzero
temperatures and ice covered for six months (from November to May) (Wroblewski et.
al., 2006). This might lead a low primary production rate in Gilbert Bay due to the
limitation of the light. Nutrient analyses of water samples indicate that the major source
of nutrients in the bay is not from terrestrial runoff in the summer (Copeman and Parrish,
2003). Cardoso et al. (2004) estimated that the primary production rate of Labrador Shelf
ranges from 212.4 g C m-2 year-1 in 1970s to 199.3 g C m-2 year-1 in the 1990s. Harrison
and Parsons (2000) calculated the primary production rate of North Sea of 200 g C m-2
year-1. Based on these studies, we set the primary production in Gilbert Bay ranges form
150 g C m-2 year-1 to 250 g C m-2 year-1. The overall fish trophic level of Gilbert Bay was
averaged from diet composition data of domain fishes in Gilbert Bay. Six main fish
species (the biomass greater than 5% of total biomass in the 2004 survey) which involved
in this calculation are: Arctic char, Atlantic cod, Greenland cod, American sand lance,
shorthorn sculpin, and winter flounder. Some of these data were directly from the fish
diet study in the bay, such as Atlantic cod. Stomach contents data (Morris, 2000) in
different times from different part of the bay were averaged for this species (Figure 3).
However, others were picked from the nearest available data off Gilbert Bay in FishBase
(www.fishbase.org). The equation from Pauly et al. (2001) was used to calculate the
individual fish trophic level (TLi) of these six species:
n
TLi = 1 + ∑ DCij TL j
j =1
where i is the predator, j is the prey, Dcij is the proportion of diet composition.
In the survey of fish fauna during 2004, 15 families and 25 species were collected
from Gilbert Bay (Wroblewski et al., 2006). The survey also shows that besides Atlantic
cod (Gadus morhua), rock cod (Gadus ogac) are also abundant in Gilbert Bay. In
addition to estimate the finfish carrying capacity, we also partitioned the theoretical total
fish production based on the information from 2004 standard survey study. To convert
the fish length to biomass, the length-weight relationships were derived from FishBase
(www.fishbase.org) and Morris et al. (2002).
Though a great number of capelin were caught during the survey, consider that
they came to the bay for spawn and only stay for a short time, and it was the first capelin
spawning event in approximately ten years in Gilbert Bay, we didn’t include it either in
trophic level or partition of fish production calculation.
RESULTS AND DISCUSSION
The 2004 standard survey shows that Atlantic cod contributes 34% of the total
fish biomass (by weight) caught in Gilbert Bay. The second largest group is Greenland
cod (18%). Arctic char, American sand lance, shorthorn sculpin, and winter flounder
have the proportions of 16%, 5%, 10%, and 9%, respectively. Figure 4 shows the
proportions by weight of the main fish species in Gilbert Bay in 2004 standard survey. As
the dominant species has the biggest effect in overall trophic level calculation, we mainly
examined Atlantic cod trophic level based on the stomach contents data from Morris et al.
(2002) on biology of the resident cod population in Gilbert Bay. Most likely the Atlantic
cod in the bay primarily feed on benthic invertebrates (Morris, 2000). Not as the Atlantic
cod in other area, Gilbert Bay cod contents very small portion of the diet on fish. Gosse
and Wroblewski (2004) found that the reddish brown to golden colouration of Gilbert
Bay cod is contributed by carotenoid-rich invertebrate diet, and suggests the cod has been
feeding at low trophic levels. These likely indicate a lower trophic level of the bay cod,
therefore a lower overall trophic level in Gilbert Bay than in the coastal area. From our
calculations, the trophic level of the bay cod is estimated as 3.3, which is far lower than
4.0 for the North Sea area (FishBase).
The mean trophic level of Gilbert Bay fishes is 3.3. This number is slightly lower
than 3.4 in coastal ecosystems (Pauly et. al., 1998; Harrison and Parsons, 2000). The
trophic levels of the six main species were shown in Table 1.
Fish Name
Arctic char
Atlantic cod
Greenland cod
American sand lance
short horn sculpin
winter flounder
Number
caught
23
73
291
3500
152
34
mean SL
(cm)
36
36.27
15.26
8.35
13.5
18.41
Total weight
(g)
17168.1
37978.1
20124.0
5637.9
11263.3
9377.2
Percentage
(%)
16
34
18
5
10
9
Trophic
level
4.3
3.3
3.5
3.2
3.1
4.4
Table 1: The proportions by weight of the main fish species caught during 2004 standard survey
and their trophic levels. Data sources: Morris et al. (2002) for Atlantic cod and FishBase
(www.fishbase.org) for the rest of the species.
From the primary production and fish production equation, therefore, we get the
annual fish production in Gilbert Bay ranges from 413 tons to 688 tons per year
considering different input parameters (Figure 5). So far, 25 fish species have been found
in Gilbert Bay (Wroblewski et al., 2006). Among these species, four are apparently yearround residents: Atlantic cod, Greenland cod, sculpin, and winter flounder (Wroblewski
et al., 2006). The partitions of the theoretical total annual fish production for the main
fish species in Gilbert Bay are shown in Table 2.
Fish Name
Atlantic herring
Atlantic salmon
Arctic char
Atlantic cod
Greenland cod
American sand lance
short horn sculpin
winter flounder
Proportion over total weight
(%)
3
3
16
34
18
5
10
9
Annual fish production
(tons)
12-21
12-21
66-110
140-234
74-124
21-34
41-69
37-62
Table 2: The estimated annual production of the main fish species in Gilbert Bay.
Generally cod average 2 to 3 kg in weight (Underwater world-Atlantic cod, 2004).
However, as the Gilbert Bay cod has a slow growth rate and small length-at-age (Morris,
2000; Morris and Green, 2002), we assumed the average body weight of 2000 g. Then we
estimate the carrying capacity of the bay cod in Gilbert Bay ranges from 427 tons to 711
tons. Presently, the Atlantic cod standing biomass in Gilbert Bay is estimated at 70 tons
(Morris, et al., 2003), which is only 1/8 of the carrying capacity. According to the
Schaefer model (1954), the maximum sustainable yield could be reached when the
biomass equals to half of its carrying capacity. Therefore, we might expect the total
Atlantic cod biomass of approximate 250 tons to get the MSY in Gilbert Bay.
This study used bottom-up ecosystem trophic dynamics to determine the fish
production. There are many factors can influence this calculation, such as trophic level,
transfer efficiency, primary production rate, and fraction of primary production that is
potentially available to mesoplankton. The values of these factor are various from one
ecosystem to another. Furthermore, a fluctuation of +10% was placed on initial parameter
values (trophic level=3.3, transfer efficiency=15%, primary production rate=190 g C m-2
year-1) to compare the sensitivity of the influence among the three common factors:
trophic level, transfer efficiency, and primary production rate (Table 3).
Annual fish
production
Trophic
level
2.97
3.3
3.63
TE=13.5%
Primary production
rate (g C m-2 year-1)
171
190
209
715
794
874
369
410
451
191
212
233
TE=15%
Primary production
rate (g C m-2 year-1)
171
190
209
880
978
1075
470
523
574
252
279
307
TE=16.5%
Primary production
rate (g C m-2 year-1)
171
190
209
1061 1179
1297
586
651
716
323
359
395
Table 3: The variation of annual fish production based on different parameters. A fluctuation of
+10% was put on to the initial values of the three common factors: trophic level (3.3),
transfer efficiency (15%), and primary production rate (190 g C m-2 year-1).
The average changes on annual fish production that the fluctuation of +10% on
TL, ET, and primary production rate could make is 700, 258, and 119 tons respectively.
The effect of the trophic level upon the estimation of annual fish production is 5.9 and 2.7
times higher than that of primary production rate or transfer efficiency. This may indicate
that in Gilbert Bay the changes in TL play a more important role in estimating the
carrying capacity than changes in ET and primary production rate.
Again, this is the first-order calculation of carrying capacity in Gilbert Bay. By
the limits of
information, we either made some assumption or gave a possible range to
those uncertain factors. For example, the trophic levels of the fish species other than
Atlantic cod are assumed to be the same as the nearest available values, the primary
production rate is in the range of 150 g C m-2 year-1 and 250 g C m-2 year-1, and so on. To
get a better understanding of this ecosystem, further observation and monitoring needs to
be done, such as more data on nutrients and chlorophyll-a concentration, biological
characteristics of other resident fish species, benthic habitats, and water circulation.
Gilbert Bay is the first Marine Protect Area in the subarctic coastal zone of
eastern Canada. The announcement of MPA represents the beginning of the action plan
of protecting the local cod population and its habitat. The better we know about our
fisheries, the better we can expect to manage it. Continuous research and monitoring
within MPA can provide the stakeholders and management committee a better
understanding of the local ecosystem so as to produce efficient management plan. It
could be a real practice area for our management plans to be applied to a larger scale as
well.
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Figure 1: Map of Gilbert Bay and sample sites. Source: Modified from Wroblewski et al.
(2006).
Figure 2: Three zones of Gilbert Bay MAP.
Gastropods
3%
Polycheate
5%
Fish
7%
Sea
cucumber
2%
Bivalve
2%
Crab
18%
Sea urchins
2%
Shrimp and
mysids
19%
Amphipods
10%
Brittle star
13%
Scallop
19%
Figure 3: Averaged major food types found in Atlantic cod stomachs from June-August 1998 and
May 1999 (Morris, 2000).
Other
2%
winter flounder
8%
short horn
sculpin
10%
Atlantic herring
3%
Atlantic salmon
3%
Arctic char
16%
American sand
lance
5%
Greenland cod
18%
Atlantic cod
34%
Figure 4: The proportion by weight of the main fish species in Gilbert Bay in the 2004
standard survey.
700
Fish production (tons/year)
600
500
400
300
200
100
0
171
190
209
Prim ary production rate (g C /squ m eter/year)
-2
Figure 5: Annual fish production with primary production ranges from171 g C m year
-2
-1
-1
to 209 g C m year (Trophic level=3.3, transfer efficiency=15%).
Changes of annual primary
production (tons)
750
600
450
300
150
0
TL (2.97-3.63)
P (171-209)
TE (0.135-0.165)
Changes of factors (from -10% to +10%)
Figure 6: The average changes of annual primary production related to the various parameters.