YIELDS, BYCATCH AND DISCARDS IN THE MULLUS SURMULETUSGILLNET FISHERY OFF
SOUTHEASTERN MALLORCA (WESTERN MEDITERRANEAN)
Xavier Mas*, Raquel Gońi and José Luis Fernández
IEO-Centre Oceanogrŕfic Balears, Palma de Mallorca, Spain - * javier.mas@ba.ieo.es
Summary
This is the first of a series of studies of the artisanal fisheries of the Balearic Islands (Western Mediterranean) that aim to evaluate their
efficiency, yields and ecosystem impacts. Here we present results of the striped red mullet (Mullus surmuletus)gillnet fishery carried out
during 1999-2001 off Southeastern Mallorca Island. The efficiency of the fishery was high with commercialised species making up 99%
of the total catch in number. However, the selectivity of the fishery was low and over 40 bycatch species of fishes and molluscs were subject
to unaccounted fishing mortality.
Key words: Bycatch, discards, gillnet, fishing impacts, Mullus surmuletus
Rapp. Comm. int. Mer Médit., 37,2004
397
Introduction
One of the ecological costs of fishing is the incidental catch of
organisms which are not target of the fisheries (1,2). Many current
fishing methods are highly indiscriminate and result in the catch of
non-target species or of undesirable sizes of target species which are
then discarded back to the water. Knowledge of bycatch and discard
practices in large-scale fisheries has grown in parallel with this aware-
ness(2,3) and in the Western Mediterranean studies have focused on
bottom trawl fisheries (4). Artisanal fisheries are generally perceived
as low impact, highly efficient fisheries that generate few if any dis-
cards. The prevalence of artisanal fisheries in the Western
Mediterranean, and in the Balearic Islands in particular, has prompted
a series of studies aimed at assessing their efficiency and ecosystem
impacts by investigating the relationship of the yield of target and non
target organisms as well as the bycatch and discard practices. Here we
present the results of the first study conducted on the striped red mul-
let (Mullus surmuletus)gillnet fishery off Southeastern Mallorca
Island (Western Mediterranean).
Material and methods
Data were collected on board artisanal vessels participating in the
autumn M. surmuletusgillnet fishery. The fishery takes place at sun-
set and at sunrise when gillnets are set over Posidonia/sand substrates
for periods of 2-3 hours. The stretched mesh size of the netting is 35-
40mm and gillnets are 500-1000m long.
Data from 48 valid fishing operations sampled randomly during the
1999-2001 seasons were used in the study. Information was collected
on fishing effort (length of net, fishing time) and on the retained and
discarded catch. For each species we calculated the yield (expressed
as the number of individuals caught in 500m) in the retained and dis-
carded fractions, and their percentage contribution to the catch. To
assess the fishery and ecological efficiency of the fishery, for each
fishing set the following indices were calculated: a) target species
index= number of retained M. surmuletus/total number of specimens
caught; b) fishery index= number of specimens retained/total number
of specimens caught; c) bycatch index= number of bycatch speci-
mens/number of M.surmuletuscaught; and d) discard index= number
of discarded specimens/number of M.surmuletuscaught.
Results and discussion
Forty-two species were caught (39 fishes and 3 molluscs), 40 of
which were retained in at least one occasion (commercial bycatch).
M.surmuletusmade up 47.7% of the catch in number and the mean
yield was 38.0 (
±
5.2 SE). The commercial bycatch made up 50.9% of
the catch in number and the mean yield was 44.6 (
±
7.9 SE). The main
bycatch species were the fishes Diplodus annularis, Spicara maena,
Diplodus vulgaris, and Serranus scriba, all of commercial value.
Twelve species were discarded at least in one occasion, and the dis-
carded fraction was 1.4% in number. M. surmuletuswere discarded in
19% of the fishing sets and made up the largest fraction of the discards
(42.8% in number). Reasons for discarding were: damages caused by
bites of dolphins or other organisms (e.g. M. surmuletus, Pagrus
pagrus, Symphodus tinca, Scorpaena notata), low or nil market value
(e.g. Synodus saurus, Sarpa salpa, Chromis chromis)and size below
the legal limit (Epinephelus costae and Scorpaena scrofa).
The target species index was 0.46 (
±
0.03 SE), indicating a relative-
ly high efficiency of the fishery for M. surmuletuswhen compared
with other artisanal fisheries, such as cuttlefish trammel net fishery off
Greece where cuttlefish made up 31% of the catch(5). The fishery
index was 0.99 (
±
0.003 SE) and shows that overall thefishery is high-
ly efficient and produces low waste. The estimated bycatch (2.3
±
0.5
SE) and discard (0.03
±
0.01 SE) indices illustrate that on average for
every M. surmuletuscaught 2.3 non-target organisms must be fished
of which 0.03 are discarded. Both in absolute terms and when com-
pared with discard rates of 25% in bottom-trawl fisheries in the
Western Mediterranean (4), or of 15-25% in gillnet and longline fish-
eries in the Northeast Atlantic (6), these values appear very low.
The good outlook of this fishery in terms of its low discard rates
should not hide the fact that its species selectivity is low as the fishery
impacts 41 bycatch species of fishes and molluscs. These species are
subject to unknown and unaccounted levels of fishing mortality while
little to nothing is known about the status of their populations. Further
assessments of the efficiency and ecological impacts of artisanal fish-
eries will consider the resilience of the bycatch species and strive to
incorporate the impacts on taxonomic groups that have no market
value and are generally not considered in these studies.
References
1-Hall, S.J. 1999. The effects of fishing on marine ecosystems and
communities. Fish Biology and Aquatic Resources Series I. Blackwell
Science. 274 p.
2-Gońi, R. 1998. Ecosystem effects of marine fisheries: An overview.
Ocean & Coastal Management, Vol. 40: 37-64.
3-Alverson, D.L., Freeberg, M.H., Murawski, S.A., Pope, J.G. 1994. A
global assessment of fisheries bycatch and discards. FAO Fisheries
Technical Paper 339. 235 p. Roma.
4-Carbonell, A., Martin, P., de Ranieri, S. and WEDIS team. 1998.
Discards of the Western Mediterranean trawl ?eets. Rapp. Comm. Int. Mer
Médit., 35: 392-393.
5-Petrakis, G. and Politou, C. Y. 2001. Cuttlefish trammel net “metier” in
Greece. Rapp. Comm. Int. Mer Médit.,36: 309.
6-Pérez, P., Pereda, P., Uriarte, A., Trujillo, V., Olaso, Y., Lens, S. 1995.
Discards of the Spanish ?eet in ICES Divisions. Study contract EU-DG
XIV. Ref. No. PEM/93/005.
