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HCS 149
ISATEC
Comparative analysis of the community structure and trophic relations of the Peruvian hake
Merluccius gayi peruanus and its by-catch of the years 1985 and 2001
Michael Ballón
Introduction
The shapes of the size spectra of
both years follow the hake’s catch
curves of the respective year
(Fig.4). The community has
changed in one direction, which
is determined by the fishing
effects, towards the small sized
individuals.
25
20
Ln(N)
Peruvian hake preys on the most abundant species of the pelagic, demersal and mesopelagic
systems. This species capitalizes on the abundances of its preys becoming the basis of the trawl
fishery. However, the long term changes of the pelagic system (anchovy-sardine shift), the effect of El
Niño and the intense fishing have depleted the Peruvian hake stock (Wosnitza-Mendo et al., 2004) to such
an extend that the fishery was closed in September of 2002, the hake stock was declared overfished and
new regulations were implemented to allow for its recovery.
Nevertheless trawl fishing is a non-selective method and affects other species apart from hake. The
continued removal of older and larger individuals from a demersal fish community, not only affects the
abundance and size structure of the community but it also prompts the smaller size species to dominate
the system (Daan et al., 2005; Piet and Jennings, 2005; Bianchi et al., 2000 and Zwanenburg, 2000). As
large individuals generally feed in higher trophic levels (Pauly et al., 1998) these changes are usually
linked to changes in the community’s trophic structure.
The aim of this study is to infer changes in the north Peruvian demersal marine ecosystem by comparing
the community structure and trophic relations of the Peruvian hake Merluccius gayi peruanus and by-catch
between 1985 and 2001.
15
10
Materials and Methods
Biomass species composition
Data source
Sampling method
Biomass species composition
: surveys 1985-03-04, 2001-05-06
: random stratified (designed for hake biomass estimation)
: identification on board, swept area
5
2.0
2.9
3.3
3.6
3.9
4.1
4.2
4.4
Ln(L)
Table1.Number of trawl stations per sub-area and stratum for each survey
A
B
C
D
E
F
03°29’- 04°- 05°- 06°- 07°- 08°04°S 05°S 06°S 07°S 08°S 09°S
6
3
9
5
2
3
4
6
9
8
3
3
4
2
3
1
2
2
3
2
4
4
3
1
3
5
9
6
6
6
6
4
5
4
3
2
Stratum
(fathoms)
Survey
1985
1
2
3
1
2
3
2001
(20-50)
(50-100)
(100-200)
(20-50)
(50-100)
(100-200)
Benthos biomass
Data source
Sampling method
Biomass composition
: surveys 1985-03-04, 2001-05-06
: random stratified (designed for hake biomass estimation)
: identification on board, swept area (sub-areas C, D, E and
stratum 1 and 2 )
Community size structure
The hake size structure was based on the population size structure estimated from the length frequencies
recorded at each trawl station. The length frequency sample of each trawl was weighted to the total hake
catch of the trawl, to the stratum, to the sub-area and finally to the total covered area (see Table1).
Frequencies per length interval (1 cm) were regrouped into length intervals of 5 cm.
For the hake by-catch, weight and numbers of each fish species in each trawl were used to
construct length frequencies. Assuming isometric growth (W = 0.01*L3) (Bianchi et al., 2000), the average
individual weight of each species was transformed into length and linked to its respective number.
Proceeding in the same way as hake, length frequencies of each species were extrapolated to the total
covered area and regrouped into 5 cm interval.
The community size structure was constructed by combining the hake size structure with the hake bycatch size structure.
To see whether the level of exploitation of hake explains the community size spectra, the hake catch curve
was transformed by replacing the age groups of hake of the catch curve by their corresponding mean
lengths at age and assigned to a 5 cm interval. Then the natural logarithm of the number of individuals at
the corresponding mean length at age group was plotted against the natural logarithm of the midsize
per size class.
Mean trophic level
Hake diet came from biological samples of the years 1985-1986 and 2000-2001.
Diet spectrum by length was transformed into diet spectrum by age using average length-age keys. Diet of
the hake by-catch was estimated based on unreported feeding studies (IMARPE), publications, Fishbase,
and biological samples (IMARPE)
Mean trophic level (TLm) was estimated based on Fishbase and unreported feeding studies from
IMARPE.
TLm = ΣTLijBij / Σ Bij
Where TLm is the mean trophic level of the biomass in year j, Bij, the biomass of the species i in the year j
and TLij is the trophic level of the species i.
• The trophic levels of 41 fish species was used to estimate TLm of 1985
• The trophic levels of 43 fish species were used to estimate the 2001 TLm
Fig. 5 Diet composition, trophic level
and biomass distribution according to
hake age groups.
It can be inferred from the
comparison of the hake by-catch
diets (Fig.6 and 7) of the two years
and from its reduction in biomass
that the macrobenthic community
had a lower predation pressure in
2001 than in 1985, and that the
observed increase in prey biomass
in 2001 (Fig.8), was due to the
biomass reduction of its main
predators. Other factors could
explain the differences in the benthic
biomass between both periods too,
such as the low and high oxygen
concentration in 1985 and 2001,
respectively. Nevertheless, if the
observed increase in the benthic
biomass resulted from its release
from predation pressure, this would
imply that the niche left by the
benthic predators in 1985 was not
filled by other species in 2001,
suggesting that benthic production
is top-down controlled.
(t)
5ºS
biom ass (t)
1985
2001
PERU
Talara
B
polychaet es
6%
phytoplankton
0.4%
detritus
0.6%
Fig. 6 Average diet of hake by-catch (only fish
species) in 1985
crustaceans
19%
other
8%
fish
73%
squid
3% mollusc
0.13%
polychaetes
2%
euphausiids
2%
zooplankton
0.1%
detritus
0.96%
1985
Paita
Pta. Gobernador
800000
600000
400000
C
6 º S 0.5 Pta. La Negra
1985
200000
Mórrope
Pimentel
D
7ºS
0
300
0
Chérrepe
E
3
Biomass 10
fish
15%
mollusc
4%
2001
Pta. Sal
4ºS
100
200
crustaceans
32%
squid
1%
1200000
Pto. Pizarro
A
1985
other
13%
1000000
300
200
euphausiids
41%
Fig. 7 Average diet of hake by-catch (only fish
species) in 2001
Results and Discussion
Although hake was distributed in a
more restricted area and under
condition of lower oxygen concentration
(Fig. 2) the estimated biomass for 1985
was 400% higher than that for
2001(Fig. 1).
The drastic changes in hake
size structure due to overfishing
and the fact that the TLm of
2001 is determined mainly by
juvenile hake (Fig. 5), which feed
on euphausiids, indicate that the
demersal fish community has
experienced the “fishing down
the marine food web” (Pauly et
al., 1998) phenomenon.
POL
Hake
0.5
F
100
0
C
D
MISCE
Fig. 8 Benthos biomass of the major taxa.
Punta Chao
9ºS
B
NEMER
Salaverry
Others species
A
MOL
Chicama
8ºS
2001
CRU
E
G
F
10 º S
81 º W
Fig 1 Biomass distribution among subareas
This means that the
community has
experienced changes in
species composition and
reduction in biomass
References
Casma
Punta Lobos
Sub-areas
Virtually
all species
present in 1985 had
decreased drastically by
2001(Fig. 3). Species with
larger sizes, long life and
late maturity seem to be
the most affected.
Chimbote
80 º W
79 º W
78 º W
Bianchi, G., Gislason, H., Graham, K., Hill, L., Jin, X., Koranteng, K., Manickchand-Heileman, S., Paya´, I., Sainsbury,
K., Sanchez, F., and Zwanenburg, K. 2000. "Impact of fishing on size composition and diversity of demersal fish
communities." ICES Journal of Marine Science 57: 558-571.
77 º W
Fig2. Area of the continental shelf with oxygen level
higher than 0.5ml/L Green line 1985, red line 2001
Daan, N., Gislason, H., Pope, J. G., and Rice, J. C. 2005."Changes in the North Sea fish community: evidence of
indirect effects of fishing?" ICES Journal of Marine Science 62: 177-188.
Prionotus stephanophrys
Mustelus whitneyi
Myliobatis chilensis
Seriolella violacea
Trachurus murphyi
Sciaena deliciosa
Genypterus maculatus
Paralabrax humeralis
Larimus pacificus
Brotula clarkae
Pontinus sierra
Paralonchurus peruanus
Hippoglossina macrops
Galeichthys peruvianus
Dosidicus gigas
Loligo gahi
Galeorhinus galeus
Cynoscion analis
Stromateus stellatus
Engraulis ringens
Myliobatis peruvianus
Echinorhinus cookei
Ctenosciaena peruviana
Peprilus medius
Pauly, D., Christensen, V., Dalsgaard, J., Froese, R., and Torres Jr., F. 1998. "Fishing down the marine food webs."
Science 279: 860-863.
Piet, G. J., and Jennings, S. 2005. "Response of potential fish community indicators to fishing." ICES Journal of
Marine Science 62: 214-225.
1985
Wosnitza-Mendo, C., R. Guevara-Carrasco and M. Ballón, 2004. Possible causes of the drastic decline in mean
length of Peruvian hake in 1992. Bol. Inst. Mar Perú 21 (1-2): 1-26.
2001
Zwanenburg, K. C. T. 2000. "The effects of fishing on demersal fish communities of the Scotian Shelf." ICES Journal
of Marine Science 57: 503-509.
catfish
Jumbo
squid squid
12
10
8
6
4
2
Biomass (%)
0
2
4
6
Fig. 3 Biomass of the 12 most abundant species after hake.