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Does food quality affect the conditions of the sand and common gobies from the Gulf of Gdańsk, Poland?

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Abstract

Sand and common goby specimens were collected from the costal waters in the vicinity of Sopot and Chałupy (Gulf of Gdańsk) from August to October. The relationship between the food consumed and the fish condition was investigated for both species using the Fulton and Clark factors, HSI, feeding intensity, and the index of relative importance. The results indicated that the mean values of the common goby condition factors (20-29 mm) were higher in September, the last month of reproduction. The sand gobies (30-39 mm) were characterized by lower condition factor values in September, one month after spawning. It was concluded that there is a direct link between diet composition and goby condition during spawning and in the months following it.
Oceanological and Hydrobiological Studies
Vol. XXXIV, No. 3
Institute of Oceanography (39-55) University of Gdańsk
2005
Research Article
DOES FOOD QUALITY AFFECT THE CONDITIONS OF THE SAND
AND COMMON GOBIES FROM THE GULF OF GDAŃSK, POLAND?
KATARZYNA WALIGÓRA BOREK , ILONA ZŁOCH, MARIUSZ R.
SAPOTA1, MONIKA FIJAŁKOWSKA, KAROLINA FORYCKA
Department of Marine Biology and Ecology, Institute of Oceanography
University of Gdańsk, al. Marszałka Piłsudskiego 46
81-378 Gdynia, Poland
e-mail:1 ocems@univ.gda.pl
Key words: sand goby, common goby, Gulf of Gdańsk, fish condition, feeding
Abstract
Sand and common goby specimens were collected from the costal waters in the vicinity of
Sopot and Chałupy (Gulf of Gdańsk) from August to October. The relationship between the food
consumed and the fish condition was investigated for both species using the Fulton and Clark
factors, HSI, feeding intensity, and the index of relative importance. The results indicated that the
mean values of the common goby condition factors (20 29 mm) were higher in September, the
last month of reproduction. The sand gobies (30 39 mm) were characterized by lower condition
factor values in September, one month after spawning. It was concluded that there is a direct link
between diet composition and goby condition during spawning and in the months following it.
K. Waligóra Borek, I. Złoch, M. Sapota, M. Fijałkowska, K. Forycka
40
INTRODUCTION
The sand goby, Pomatoschistus minutus (Pallas 1770), and the common
goby, Pomatoschistus microps (Kröyer 1840), are two of the most abundant fish
species in the estuaries, lagoons, and shore waters of Europe (Salgado et al.
2004). The geographical distribution of the common goby ranges from the coast
of Norway to the Gulf of Lion in the Mediterranean Sea (Boucherereau and
Goulorget 1997), while that of the sand goby ranges from the coast of Norway
to the west coast of the Black Sea (Boucherereau et al. 1989). These species
prefer exposed sandy, exceptionally muddy, or bottom habitats overgrown with
marine plants (Żmudziński 1990). Shallow costal waters are environments with
highly variable biotic and abiotic conditions. While physical parameters like
temperature, salinity, and pH have an impact on fish life functions, so does the
diversity of their food. The sandy bottom ecosystem is an important habitat for
many fish species, and it provides suitable conditions for developing young and
small fishes away from larger predators. The sand goby reaches very high
densities in the littoral zone, so it can be an important object of prey (Hesthagen
1977).
It has been observed that the sand and common gobies coexist in some
shallow waters on the Polish coast. These species are closely related and
morphologically similar; moreover, they consume approximately the same food
(Edlund and Magnhagen 1981). Several studies on Gobiidae feeding have been
published, but they were either conducted in other regions of occurrence or as
laboratory experiments (Edlund and Magnhangen 1981, Magnhagen and
Wiederholm 1982, Aarnio and Bondorff 1993).
The aim of the present study was to determine whether there is a
relationship between the diet and condition of P. minutus and P. microps in the
Gulf of Gdańsk.
MATERIAL AND METHODS
The study was conducted in the costal waters of the Gulf of Gdańsk. The
sand goby specimens were collected in August 2002 (last month of spawning)
and in September 2002 (first month following spawning). Specimens of
common goby were collected from September 2002 (last month of spawning) to
October 2003 (first month following spawning). P. microps specimens were
caught in Sopot (in September) and Chałupy (in October), while all of the
P. minutus individuals were collected in Sopot. The material was collected at a
depth of 1 m using a towing-net with a 2 m opening. The distance of the hauls
was approximately 100 m. Following capture, the gobies were preserved in a
Does food quality affect the conditions of the sand and common gobies ...
41
4% buffered formaldehyde solution. In the laboratory, the total length of the
fish was measured to the nearest 1 mm. The fish were also weighed, with and
without viscera, to the nearest 0.0001 g. The livers and stomachs were weighed
to the nearest 0.0001 g. The stage of stomach fullness was determined using the
following formula:
stomach fullness index =
%100
fish ofweight food ingested ofweight
The stomach contents were identified to the lowest possible taxonomic
level. The stomach contents were counted and measured under a
stereomicroscope. Prey wet weight was determined using the length-weight
relationship (Berestovsky et al. 1989, Witek 1995). These data were analyzed in
terms of frequency of occurrence, quantity, and weight in fish stomachs, and
were presented as the index of relative importance (IRI) (Pinkas et al. 1971).
Individual data categories were expressed as percentages:
IRI = (%N + %W) · %O
where:
%N numerical percentage of a food item in the stomachs;
%W percentage by volume of a food item in the stomachs;
%O frequency of occurrence of a food item.
The relationship between the relative importance of a given prey item and
its energetic value was assessed and considered in light of the research by Witek
(1995).
The following indexes were calculated: Clark condition factor (relationship
between weight without viscera and total length); Fulton condition factor
(relationship between total weight and total length); hepatosomatic index (HSI)
(dependence between liver weight and total fish weight) (Ricker 1975). The
factors and indexes are not presented in the same fish length classes for both
species in each month because individuals were scarce in the investigated area.
RESULTS
Fulton factor
The highest Fulton factor values were attained by the common goby from
the 30-39 mm length class in October. This is the first month following
K. Waligóra Borek, I. Złoch, M. Sapota, M. Fijałkowska, K. Forycka
42
spawning. The sand goby attained the highest values in the 40-49 mm length
class during the last month of spawning. The condition factor usually increased
with fish length. The opposite trend was observed only in the case of the
common goby during the last month of spawning, and only for these same fish
did the Fulton factor decrease with fish length (fig. 1).
P. minutus August
median
25%-75%
min.-max.
30-39 mm 40-49 mm 50-59 mm
length class
0.0005
0.0006
0.0007
0.0008
0.0009
0.0010
0.0011
0.0012
Fulton factor
P. minutus September
median
25%-75%
min.-max.
P. microps September
median
25%-75%
min.-max.
20-29 mm 30-39 mm 40-49 mm
length class
0.0005
0.0006
0.0007
0.0008
0.0009
0.0010
0.0011
0.0012
Fulton factor
P. microps October
median
25%-75%
min.-max.
Fig. 1. Changes of Fulton factor values in fish length classes
Clark factor
The values of the Clark factor usually increased with fish length. September
is the only exception; the condition factor for P. microps decreased with fish
length. The highest values were achieved by both the common goby (20-29 mm
length class) and the sand goby (50-59 mm length class) in September (Fig. 2).
Hepatosomatic index (HSI)
The HSI increased in all months with fish length for the sand goby, while
for the common goby it only did so in October. P. microps achieved the highest
HIS in October in the 30-39 mm length class. The lowest values were in
September for the common goby in the 30-39 mm and 40-49 mm length classes
(Fig. 3).
Does food quality affect the conditions of the sand and common gobies ...
43
P. minutus August
median
25%-75%
min.-max.
30-39 mm 40-49 mm 50-59 mm
length class
-0.0002
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
Clark factor
P. minutus September
median
25%-75%
min.-max.
30-39 mm 40-49 mm 50-59 mm
length class
-0.0002
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
Clark factor
P. microps September
median
25%-75%
min.-max.
20-29 mm 30-39 mm 40-49 mm
length class
-0.0002
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
Clark factor
P. microps October
median
25%-75%
min.-max.
10-19 mm 20-29 mm 30-39 mm
length class
-0.0002
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
Clark factor
Fig. 2. Changes of Clark factor values in fish length classes
P. minutus August
median
25%-75%
min.-max.
30-39 mm40-49 mm50-59 mm
lenght class
0
1
2
3
4
5
6
7
8
HSI index
P. minutus September
median
25%-75%
min.-max.
30-39 mm40-49 mm50-59 mm
lenght class
0
1
2
3
4
5
6
7
8
HSI index
P. microps September
median
25%-75%
min.-max.
20-29 mm30-39 mm40-49 mm
lenght class
0
1
2
3
4
5
6
7
8
HSI index
P. microps October
median
25%-75%
min.-max.
10-19 mm20-29 mm30-39 mm
length class
0
1
2
3
4
5
6
7
8
HSI index
Fig. 3. Changes of HSI values in fish length classes
K. Waligóra Borek, I. Złoch, M. Sapota, M. Fijałkowska, K. Forycka
44
Feeding intensity
In August 2001, the largest food intake was exhibited by the sand gobies
from the 40 49 mm length class. In the following month, the youngest
individuals of P. minutus fed the most intensively. In the case of P. microps, the
highest rate of food intake was observed for individuals from the 20 39 mm
length classes. In October 2003, at Chałupy station, the longest common goby
individuals had the highest value of stomach fullness index (Fig. 4).
P. minutus August
median
25%-75%
min.-max.
30-39 mm 40-49 mm 50-59 mm
0.0
0.1
0.2
0.3
0.4
0.5
P. minutus September
median
25%-75%
min.-max.
30-39 mm 40-49 mm 50-59 mm
0.0
0.1
0.2
0.3
0.4
0.5
P. microps September
median
25%-75%
min.-max.
20-29 mm 30-39 mm 40-49 mm
0.0
0.1
0.2
0.3
0.4
0.5
P. microps October
median
25%-75%
min.-max.
10-19 mm 20-29 mm 30-39 mm
0.0
0.1
0.2
0.3
0.4
0.5
Fig. 4. Changes of seasonal feeding intensity in gobies by length classes
Food composition of P. minutus
Numerically, calanoids dominated the sand goby diet in all fish length
classes in August 2001. Copepod eggs, chironomids, and sand goby young were
the most frequently found prey items in the food of all the
investigated fish length classes of P. minutus. In the diet of the sand goby from
the 30 49 mm length class, amphipods were the dominants in terms of
biomass, but in the stomachs of the longest specimens, the prey included young
individuals of its own species. For smaller P. minutus (from the 30 - 39 and 40 -
49 mm length classes), the most important prey (%IRI) were calanoids, whereas
for the largest ones (exceeding 50 mm) the most significant prey object was
P. minutus (Table 1).
Does food quality affect the conditions of the sand and common gobies ...
45
Food composition in different length classes of P. minutus in August 2001
Sand goby length classes
30-39 mm
40-49 mm
50-59 mm
Prey item
%O
%N
%W
%IRI
%O
%N
%W
%IRI
%O
%N
%W
%IRI
Amphipoda
20.00%
0.88%
0.48%
0.29%
5.00%
0.30%
24.28%
2.97%
-
-
-
-
Bathyporeia pilosa
24.00%
1.06%
94.56%
24.33%
5.00%
1.50%
68.85%
8.51%
-
-
-
-
Calanoida
84.00%
76.81%
0.33%
68.71%
60.00%
33.83%
0.07%
49.23%
50.00%
25.00%
0.32%
9.85%
Copepoda
28.00%
4.60%
0.014%
1.37%
30.00%
3.59%
0.01%
2.62%
-
-
-
-
Gammarus sp
-
-
-
-
10.00%
0.60%
0.21%
0.20%
-
-
-
-
Harpacticoida
8.00%
0.35%
0.00012%
0.03%
15.00%
9.28%
0.0012%
3.37%
25.00%
4.17%
0.0029%
0.81%
Insecta
-
-
-
-
15.00%
0.90%
0.23%
0.41%
-
-
-
-
Eggs
4.00%
0.18%
0.000035%
0.01%
10.00%
0.60%
0.00026%
0.14%
25.00%
8.33%
0.014%
1.62%
Copepoda eggs
24.00%
7.61%
0.00014%
1.94%
25.00%
39.22%
0.00024%
23.73%
25.00%
4.17%
0.00045%
0.81%
Mysidacea
-
-
-
-
5.00%
0.30%
0.02%
0.04%
25.00%
4.17%
1.53%
1.11%
Nematoda
4.00%
0.18%
0.00005%
0.01%
5.00%
0.60%
0.00006%
0.07%
-
-
-
-
Neomysis integer
4.00%
0.35%
0.63%
0.04%
-
-
-
-
-
-
-
-
Pomatoschistus minutus
20.00%
1.59%
2.86%
0.95%
30.00%
2.40%
1.43%
2.78%
75.00%
20.83%
90.38%
64.91%
Mysis mixta
4.00%
0.18%
0.29%
0.02%
-
-
-
-
-
-
-
-
Balanus improvisus
4.00%
0.18%
0.00008%
0.01%
-
-
-
-
-
-
-
-
Chironomidae
40.00%
4.96%
0.015%
2.11%
30.00%
3.59%
0.0043%
2.61%
75.00%
25.00%
0.17%
14.69%
Cladocera
8.00%
0.35%
0.00036%
0.03%
-
-
-
-
-
-
-
-
Pisces
12.00%
0.53%
0.63%
0.15%
-
-
-
-
-
-
-
-
Pomatoschistus sp.
4.00%
0.18%
0.19%
0.02%
25.00%
1.80%
2.21%
2.42%
50.00%
8.33%
7.59%
6.20%
Bosmina sp.
-
-
-
-
5.00%
0.30%
0.00011%
0.04%
-
-
-
-
Cestoda
-
-
-
-
5.00%
0.30%
0.00003%
0.04%
-
-
-
-
Polychaeta
-
-
-
-
10.00%
0.60%
2.6484%
0.79%
-
-
-
-
Pontoporeia affinis
-
-
-
-
5.00%
0.30%
0.0466%
0.04%
-
-
-
-
Number of stomachs
25
20
4
K. Waligóra Borek, I. Złoch, M. Sapota, M. Fijałkowska, K. Forycka
46
Table 2
Food composition in different length classes of P. minutus in September 2001.
Sand goby length classes
30-39 mm
40-49 mm
50-59 mm
Prey item
%O
%N
%W
%IRI
%O
%N
%W
%IRI
%O
%N
%W
%IRI
Amphipoda
27.59%
3.50%
34.14%
23.56%
16.00%
0.90%
16.88%
4.45%
37.50%
1.09%
28.67%
13.87%
Annelida
-
-
-
-
2.00%
0.11%
0.00%
0.00%
-
-
-
-
Bathyporeia pilosa
31.03%
3.85%
30.41%
24.13%
40.00%
3.95%
46.41%
31.54%
25.00%
1.45%
26.27%
8.62%
Calanoida
17.24%
4.90%
0.45%
2.09%
14.00%
1.13%
0.14%
0.28%
-
-
-
-
Copepoda
-
-
-
-
20.00%
2.15%
0.07%
0.69%
37.50%
22.10%
0.41%
10.50%
Gammarus sp
-
-
-
-
4.00%
0.23%
5.12%
0.33%
-
-
-
-
Harpacticoida
34.48%
25.87%
0.14%
20.36%
32.00%
9.72%
0.09%
4.91%
25.00%
0.72%
0.01%
0.23%
Heterotanais oerstedti
-
-
-
-
2.00%
0.11%
1.07%
0.04%
-
-
-
-
Hydrobia sp.
-
-
-
-
2.00%
0.11%
0.56%
0.02%
-
-
-
-
Insecta
-
-
-
-
2.00%
0.11%
1.77%
0.06%
-
-
-
-
Eggs
-
-
-
-
2.00%
0.23%
0.02%
0.01%
-
-
-
-
Copepoda eggs
13.79%
55.94%
0.01%
17.51%
42.00%
78.42%
0.09%
51.62%
62.50%
72.46%
0.05%
56.34%
Mysidacea
10.34%
1.05%
3.15%
0.99%
10.00%
0.56%
2.37%
0.46%
-
-
-
-
Nematoda
10.34%
1.75%
0.01%
0.41%
4.00%
0.68%
0.00%
0.04%
-
-
-
-
Neomysis integer
17.24%
1.75%
22.94%
9.66%
16.00%
1.13%
20.10%
5.32%
25.00%
1.09%
19.41%
6.37%
Pomatoschistus minutus
-
-
-
-
2.00%
0.11%
2.62%
0.09%
-
-
-
-
Pontoporeia femorata
-
-
-
-
4.00%
0.23%
0.83%
0.07%
-
-
-
-
Praunus flexuosus
-
-
-
-
2.00%
0.11%
1.84%
0.06%
-
-
-
-
Gastropoda
3.45%
0.35%
0.17%
0.04%
-
-
-
-
12.50%
0.36%
0.87%
0.19%
Macoma balthica
3.45%
0.35%
2.95%
0.26%
-
-
-
-
-
-
-
-
Mysis mixta
6.90%
0.70%
5.62%
0.99%
-
-
-
-
-
-
-
-
Mesopodopsis slabberi
-
-
-
-
-
-
-
-
12.50%
0.36%
8.84%
1.43%
Pygospio elegans
-
-
-
-
-
-
-
-
12.50%
0.36%
15.46%
2.46%
Number of stomachs
29
59
8
Does food quality affect the conditions of the sand and common gobies ...
47
In September 2001, the most abundant food objects in the diet of the sand
goby from all length classes were Copepoda eggs. These food items were also
found more often than not in the stomachs of individuals longer than 40 mm.
The exceptions were fish from the 30 - 39 mm length class, because their diet
consisted of numerous harpacticoids. Taking into account the prey biomass, the
diet of all sand goby individuals contained a high percentage of amphipods
(mainly Bathyporeia pilosa). The highest values of the relative importance
index were noted for B. pilosa (sand goby under 39 mm) and for Copepoda eggs
(sand goby exceeding 40 mm) (Table 2).
Table 3
Food composition of P. microps in different length classes in September 2001.
Common goby length classes
20-29 mm
30-39 mm
40-49 mm
Prey item
%O
%N
%W
%IRI
%O
%N
%W
%IRI
%O
%N
%W
%IRI
Amphipoda
7.69%
0.88%
30.81%
7.41%
5.56%
0.54%
9.09%
1.68%
-
-
-
-
Bathyporeia pilosa
15.38%
1.77%
61.62%
29.65%
22.22%
2.69%
45.46%
33.60%
22.22%
1.60%
49.34%
28.60%
Calanoida
7.69%
1.77%
0.25%
0.47%
5.56%
5.91%
1.22%
1.24%
11.11%
6.40%
0.91%
2.05%
Harpacticoida
38.46%
23.01%
0.33%
27.29%
27.78%
17.20%
0.12%
15.11%
33.33%
14.40%
0.21%
12.31%
Copepoda eggs
15.38%
70.80%
0.07%
33.14%
11.11%
69.89%
0.01%
24.39%
22.22%
76.00%
0.06%
42.70%
Mysidacea
7.69%
0.88%
6.90%
1.82%
5.56%
0.54%
2.04%
0.45%
-
-
-
-
Nematoda
-
-
-
-
5.56%
0.54%
0.003%
0.09%
-
-
Neomysis integer
-
-
-
-
16.67%
2.69%
42.06%
23.42%
11.11%
1.60%
49.48%
14.34%
Balanus improvisus
7.69%
0.88%
0.02%
0.21%
-
-
-
-
-
-
-
-
Number of stomachs
5
9
3
Food composition of P. microps
In September 2001, Copepoda eggs were the most important prey in terms
of quantity in the dietary composition of the common goby. However, the most
frequently found food items in the common goby stomachs were harpacticoids.
The weight of B. pilosa and the undetermined Amphipoda was significant in the
food of the smallest common goby individuals. Hence, the bulk of the food
biomass of common goby longer than 30 mm not only consisted of B. pilosa,
but also of N. integer. Copepoda eggs were the most important food items
(%IRI) for the common goby from the 20 29 mm and 40 49 mm length
classes (Table 3).
The food composition at the Chałupy station was dominated quantitatively
by Harpacticoida in every common goby length class (20 - 29, 30 - 39, 40 - 49
mm). In the P. microps diet from all length classes, B. coregoni maritima and
Harpacticoida often occurred. Additionally, chironomids were common prey
items in the food composition of common goby longer than 30 mm. Polychaeta
K. Waligóra Borek, I. Złoch, M. Sapota, M. Fijałkowska, K. Forycka
48
dominated in the food biomass compared to the weight of other prey in the
stomachs of the common goby between the 30 49 mm length classes.
Comparable prey biomass values in the smallest P. microps individuals were
noted for B. pilosa. The highest values of the relative importance index were
noted for Harpacticoida in every common goby length class (Table 4).
Table 4
Food composition of P. microps in different length classes in October 2003.
Common goby length classes
20-29 mm
30-39 mm
40-49 mm
Prey item
%O
%N
%W
%IRI
%O
%N
%W
%IRI
%O
%N
%W
%IRI
Amphipoda
-
-
-
-
9.09%
0.66%
4.00%
0.46%
-
-
-
-
Bathyporeia pilosa
25.00%
1.27%
95.20%
19.20%
15.15%
1.10%
9.10%
1.68%
-
-
-
-
Bosmina coregoni maritima
100.00%
25.32%
2.38%
22.05%
69.70%
13.63%
0.15%
10.46%
66.67%
13.79%
0.03%
7.42%
Calanoida
-
-
-
-
9.09%
0.66%
0.04%
0.07%
-
-
-
-
Cestoda
25.00%
1.27%
0.03%
0.26%
3.03%
0.22%
0.0007%
0.01%
-
-
-
-
Chironomidae
25.00%
1.27%
0.17%
0.29%
57.58%
8.35%
0.26%
5.40%
100.00%
17.24%
0.15%
13.99%
Copepoda
-
-
-
-
3.03%
0.22%
0.0008%
0.01%
-
-
-
-
Cyclopoida
-
-
-
-
3.03%
0.22%
0.0238%
0.01%
-
-
-
-
Harpacticoida
100.00%
70.89%
2.22%
58.21%
93.94%
71.21%
0.26%
73.11%
100.00%
62.07%
0.04%
49.98%
Eggs
-
-
-
-
6.06%
0.44%
0.0072%
0.03%
-
-
-
-
Copepoda eggs
-
-
-
-
6.06%
2.64%
0.0006%
0.17%
-
-
-
-
Polychaeta
-
-
-
-
9.09%
0.66%
86.15%
8.59%
33.33%
3.45%
86.38%
24.10%
Gammarus locusta
-
-
-
-
-
-
-
-
33.33%
3.45%
13.40%
4.52%
Number of stomachs
4
33
3
Calorific value of prey
The present paper uses the calorific prey values from the research by Witek
(1995) (Table 5).
Table 5
Calorific values of prey items
Prey items
Organic carbon content
[gC/g m.m.]
Calorific value
[kJ/g m.m.]a
Nereis diversicolor
0.068
3.4
Pygospio elegans
0.064
3.2
Balanus improvisus
0.020
1
Bathyporeia pilosa
0.065
3.25
Gammarus sp.
0.063
3.15
Pontoporeia affinis
0.080
4
Pontoporeia femorata
0.084
4.2
Hydrobiidae
0.068
3.4
Macoma balthica
0.035
1.75
Nematoda
0.083
4.15
Harpacticoida
0.065
3.25
Mysis mixta
0.040
2
Neomysis integer
0.038
1.9
Gobiidae
0.077
3.85
Mesozooplankton crustaceans
0.064
3.2
a based on the estimation that 1 g C equals 50 kJ
Does food quality affect the conditions of the sand and common gobies ...
49
DISCUSSION
Optimalization is an important aspect of fish feeding. How can an animal
adapt its feeding to achieve the maximum energy gain, which is then used for
maintenance, growth, and reproduction? Most animals can consume a wider
range of prey than they actually do (Viherluoto 2001). An organism has to
maximize its overall energy gain. Accordingly, it has to considered whether to
invest energy in foraging for the most profitable prey or to feed on every
potential organism unconditionally and expend minimum energy on searching
and catching. The best feeding strategy would be to strike a balance between
these alternatives, and, depending on the availability of different prey, select the
best one (Landry 1981). Although large prey are energetically the best choice,
they are also difficult to catch or to ingest; thus, selecting the best feeding
techniques and foraging locations are also very important (Viherluoto 2001).
As regards foraging, fitness is understood as the rate of net energy gain.
Foraging is closely correlated with fitness because a high rate of energy gain
can decrease the amount of time spent on foraging and increase growth rate or
energy stores. It is clear that fish behave so as to maximize their rate of net
energy gain. This should lead to improved fitness; however, under some
circumstances, maximizing the rate of net energy gain will not maximize
fitness. Most fish energy resources are available for fast growth, the
maintenance of life functions, and reproduction; thus, the additional cost of
foraging directly influences survival rather than energy storage.
The food intake of fish is usually limited by food availability. After a period
of restricted feeding, fish may increase their feeding rate due to increased food
supply. These fish are often hyperphagic with higher food consumption than
those which feed continuously at the same rate (Ali and Wootton 2001). After a
prey depletion period, the fish first try to re-fill the gut. The time of re-filling
depends on gut size as its capacity changes during fish growth and may vary as
a consequence of an ontogenic shift in diet composition. In the case of the Gulf
of Gdańsk, it seems that its shallow waters provide quite a good prey supply
(Łukasiewicz 2002, Złoch 2004) and fish do not suffer from starvation. The
sand goby strategy for energy compensation appears to differ from the pattern
described above. After a limited feeding period during spawning, the sand goby
chooses high-energetic food items, such as Copepoda eggs, rather than larger
prey. Even the largest individuals of sand goby that could have preyed on larger
prey objects chose this strategy, and their condition improved continuously in
September. Contrary to increasing condition index values, the intensity of
feeding decreased. This is because the food intake index is based on food
weight. Copepoda eggs did not comprise the bulk of the sand goby food
K. Waligóra Borek, I. Złoch, M. Sapota, M. Fijałkowska, K. Forycka
50
biomass, but they were still the most important (%IRI) food item. In the first
months of the study (August and September), the mean value of food intake of
both gobies in all length classes was higher than in the following months. This
decline in feeding rate could have been caused by lower water temperatures
since exothermic fishes decrease their metabolism and level of activity as
temperatures decrease (Nikolsky 1967), and this entails a lower food intake.
The food composition of both gobies suggests that they fed on organisms
whose distribution in the environment is patchy, e.g., Neomysis integer. A patch
is understood as any group of organisms that is higher in density than the
environmental average. The utilization of a patch can be energetically beneficial
because of the greater average food intake rate (gain) and limited foraging time.
However, the advantage of this strategy for the fish is rather short-term when
considering a single patch. The rate of gain while foraging in a patch declines
because of prey depletion. In general, the foraging decision to move to another
patch is affected both by the environment and the body condition of the fish.
The observed range of goby food is broad, which means they are well
adapted to biotic changes in the environment, and they freely switch to suitable
food items. Even if they do not starve, they have a limited time in which to
obtain rich food sources that allow them to spawn. Their risk of falling prey to
larger predators may be higher than that of long-lived species. There are also
other potential costs of plasticity, such as reduced breeding, reduced growth,
and errors in appropriate behavioral responses. When food supplies are highly
variable, as in the case of coastal waters, fish may be forced to store fat for later
use. Therefore, feeding plasticity is expected to increase with environmental
variability (Komers 1997).
Male-male competition and parental care (nest preparation and guarding
offspring) are energetically costly and require good condition (Lindström 1998).
Due to this, food affects the distribution of breeding gobies. In the field, most
non-breeding gobies are in poorer body condition than nesting individuals
(Kangas 2000). Female mating costs may change over the spawning season, and
they become more choosy later in the breeding season (Forsgren 1997), which,
in the present study, was in August or September. This fact can affect the body
condition of males and females. Males are in good condition at the beginning of
the spawning season, so most of them are not rejected by females. The search
for a mate can be more costly in terms of predation risk early in the spawning
season. Hence, older goby individuals use more of their accumulated energy for
courtship. This is reflected in a decline in the hepatosomatic index of older
individuals in August. The energy budget even seems to control female choice
between dominant and non-dominant males. Sometimes, the cost of choosing a
dominant male can outweigh the benefits (Qvarnström and Forsgren 1998).
Does food quality affect the conditions of the sand and common gobies ...
51
Highly competitive males pay direct energy costs for aggressiveness that are
related to increased metabolic rates, and they could be more liable to starve.
They can compensate for these costs through the filial cannibalism of their eggs.
This was not noted in the present study because, other than those of Copepoda,
eggs were not an important food item for either goby species.
Throughout the spawning period the gobies tended to feed continuously but
with variable intensity (Złoch 2004). As the spawning season progresses, body
energy resources decline and guarding males may compensate for this by
cannibalizing their own eggs (Lindström 1998). In the present study, the gobies
did not exhibit such behavior. Both of the investigated niches probably provided
alternative high-energy food sources to help gobies survive until the end of
breeding. This can be seen in the example of the consumption of Copepoda
eggs by the common goby. This food item constituted most of the P. microps
diet in the last month of spawning (September) and its importance declined
considerably in the following month as it was no longer needed to such an
extent. However, this also could have been the result of decreased Copepoda
egg abundance in the environment due to the completion of spawning. The
decline of this goby food component requires further investigation.
In October 2004, immature common goby individuals were observed. Their
energy costs and benefits are considerably different from those of older
individuals. Due to their small size, their prey is also small and consists mainly
of Bosmina coregoni maritima and Harpacticoida. As young gobies grow, they
also feed quite frequently on Chironomidae. The choice of food by the youngest
gobies cannot be considered in terms of spawning. Their aims are different from
those of adult individuals. Larval mortality can be caused by predation and
starvation, so parts of the body needed for swimming and feeding must develop
as rapidly as possible and preferably in balance. The importance of
Bathyporeia pilosa for newly-hatched individuals of the common goby is very
high, as is that of meiofauna. However, such a high relative importance index
value is influenced by the individual biomass of larger prey. B. pilosa is not
abundant in the common goby food in all length classes, which reflects its low
availability in the environment (Kotwicki 1997).
For such short-lived species like the sand and common gobies, fast growth
seems to be a priority in the early stage of development. They need to reach the
required length to spawn in a short period as they die soon after reproduction.
(Wheeler 1969). The mean weight of common gobies in particular length
classes is almost twice that of sand gobies (Wendt 2004). Additionally, the
common goby can be considered to be better adapted to more effective growth
than the sand goby. Protection through lipid reserve levels and good body
condition are more important for the older individuals.
K. Waligóra Borek, I. Złoch, M. Sapota, M. Fijałkowska, K. Forycka
52
The Fulton and Clark condition factors for the 20 29 mm common goby
were very similar in September and October. This may indicate that these
individuals were too small to spawn. The significant decline in the
hepatosomatic index value in October shows that they used their energy stores
for fast growth and development rather than for spawning. Moreover, decreased
somatic condition in adult common gobies during warm months was not only
caused by courtship. High water temperature in the summer resulted in an
increased metabolic rate. Furthermore, food intake was lower than average
during spawning so decreased prey energy gain was insufficient to maintain
metabolic needs. This was probably the reason that the sand gobies were even
forced to feed on their own newly-hatched young in August to cover all energy
costs.
As regards the occurrence of N. integer in the food of gobies, it is clear that
this prey item is important only for larger fish individuals. Furthermore, due to
the low abundance of N. integer in the environment at the Chałupy station
(Kotwicki 1997), only the gobies from the Sopot station fed on this species. In
September, the high biomass of N. integer in the food of both gobies indicated
that, for some reason, the fish chose this prey in that particular month. It seems
that the energy value of one prey organism is insufficient to explain fish food
choice. Even if N. integer individuals do not attain a high energy value in
September, the quantity of the accumulated energy in the population per water
volume unit reaches its maximum level in that particular month (Szaniawska et
al. 1986). The N. integer biomass per water volume unit is the highest in
September.
In September 2001, the feeding intensity of the common goby decreased,
which was reflected in its lower condition. What is more, the older the common
goby were, the less important were amphipods in their food. The largest
amphipod biomass decline was observed in the food mass of the common goby
in the 20 to 39 mm length range. This resulted in the lower condition of this fish
species. This observation concurs with the findings of Jackson et al. (2002),
who concluded that the depletion of macrofaunal prey resources leads to a loss
of fitness. This is partially caused by the increased length of the search for
meiofaunal prey and a lower energetic return. Indeed, feeding on large prey
decreases foraging time, and there is also a lower risk of being predated. On the
other hand, most of the considered prey species have a patchy distribution in the
environment so the time spent searching for them should not differ
considerably. The energetic value of mesozooplankton and macrofaunal prey
also did not differ significantly. It is possible that feeding on zooplankton does
not lower fish fitness, and it may be energetically beneficial since a higher
number of small prey is available.
Does food quality affect the conditions of the sand and common gobies ...
53
The sand goby choice of food was partially different from that of the
common goby. In contrast to P. microps, the sand goby food composition
consisted of typically planktonic Calanoida. This indicates that they utilized the
water column differently from the common goby, which prefers rather
shallower waters (Wiederholm 1987). Furthermore, the B. pilosa biomass in
P. minutus food was at a rather constant value in all length classes (with the
exception of 50 mm fish in August) as was the feeding rate, but the condition of
the sand goby varied. So, again it can be concluded that prey other than
macrofaunal organisms influenced the fitness of fish. Planktonic prey is also a
good source of food because it accumulates lipids first as opposed to benthic
prey, which store carbohydrates first and then lipids (Szaniawska 1993).
In future environmental studies, the sex of gobies should be considered
separately as food abundance affects females and males differently. Males
increase their potential reproductive rate when there is a food shortage, while
females reproduce more slowly (Kvarnemo 1997).
CONCLUSION
Gobies spend their available energy stores with varying priority for
particular needs at different stages of development. Therefore, their choice of
prey items depends on food availability in the environment and fish energy
requirements. Apart from trophic niche richness, spawning period also directly
affects goby dietary composition. Goby feeding exhibits a high degree of
plasticity adjusted to current energy costs and benefits. The energy content of
prey is partially responsible for the condition of both gobies. Regardless of this,
the condition pattern of the common goby in all investigated lengths differed
from that of the sand goby.
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English revision: Jennifer Zielińska
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Pomatoschistus microps (Pisces: Gobiidae), a predatory, benthic feeder, occurs in high densities in variable, shallow, muddy coastal waters where prey availability can be unpredictable. Here, we investigate the effect and consequences of prey depletion on the body condition and nest building of adult male fish using manipulative mesocosm experiments in which the comparative value of meiofaunal and macrofaunal prey was investigated. Fish maintained in mesocosms from which macrofauna had been removed from the sediment had a significantly reduced hepatosomatic index (HSI), and were less likely to build nests than fish in mesocosms containing macrofauna and meiofauna, suggesting an indirect link between prey availability and nest quality. Reduced resource availability, in the form of scarcity of macrofauna, appears to have profound implications for the fitness of natural field populations.
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Female mate-sampling behaviour and mate choice were investigated in a natural population of sand gobies,Pomatoschistus minutusThe sand goby is a promiscuous fish with paternal care. Gravid females were released in the field and watched while they inspected potential mates. In total, 26 females were observed until they spawned. Half of the females spawned with the first male encountered. The other females inspected two or more males in sequence and never returned to a previously inspected male to mate. Females mated preferentially with males that courted intensively. The mate-sampling behaviour of the female sand gobies was most consistent with a threshold-criterion tactic, which is to be expected if search costs are important. Moreover, there was an effect of season, such that females sampled very few males early but apparently became more selective later in the breeding season. This is discussed and interpreted as an effect of varying costs and benefits of being choosy over the season.