MARINE EUTROPHICATION
My studies on fluctuations of population size of shellfish resources began in Canada, and were seen
to reflect long-term natural fluctuations in tidal regimes and temperature. When beginning work as
technical secretary to the Mediterranean Commission (GFCM), it became evident that the
productivity of some Northern Mediterranean sub-areas was not only affected by natural
environmental fluctuations, but also by a trend in runoff of nutrients from land of anthropogenic
origins in recent decades.
A negative factor in the Mediterranean trawl fishery, according to classical fisheries theory, is the
very low size at first exploitation of cohorts (Ages 0+ to 1+, with few adult fishes present in the
catches for some key species, and the lack of quota and effort control. These features all suggest
serious overexploitation. This was confirmed by the few estimates of exploitation rate then
available. However, landings, while not high, were not declining as would have been predicted
from experience elsewhere, but in many cases were increasing during the 1970’s and 80’s. A
review of landing trends in the late 1970’s-80’s also suggested this for many species.
Extensive experience by UNEP and other environmental agencies, which were alarmed by rising
nutrient inputs, had documented progressive eutrophication of many semi-enclosed seas,
especially in the Mediterranean basins. This suggested, with rising landings of small fishes, that
the lower levels of the food web were receiving greater nutrient inputs than formerly. The
Mediterranean for example, was once an oligotrophic water body outside of estuarine areas in the
early 20C, but by the 1980’s from satellite imagery, it was clear that areas such as the Gulf of
Lions and Adriatic, were receiving heavy river inputs of nutrients due to human activities on land.
In the case of the Black Sea, eutrophication of an almost-enclosed sea body was fed by a large
catchment area subject to heavy nutrient supplements. This led in the 1980-90’s to what can be
called a ‘toxic nutrient shock’ of excess nutrients due to runoff of large rivers such as the Danube
and Dnieper. This has led to a collapse of the biological system, well documented by scientists of
coastal countries, which affects almost all fishery resources. Supplemented by over-exploitation,
migratory pelagic fish ceased to arrive seasonally from the Mediterranean to the eutrophic
northern basin, reducing their top-down predatory control on small pelagic stocks. Key effects
arose from the elimination of benthic algae which formerly oxygenated shelf waters, now hypoxic.
This in turn, eliminated benthic filter feeders such as mussels which previously had removed
suspended material.
The arrival of an exotic ctenophore (Mnemiopsis) from the West Atlantic in ship ballast water, led
to its explosion in numbers, causing a collapse of small pelagic stocks in the late 1970’s-80’s. A
steady-state’ ECOPATH modeling exercise considered the possibilities for control of the outbreak,
and suggested that a predator might help control Mnemiopsis populations. This option remained
open, given that the only natural Mnemiopsis predator in its ‘home habitat’ in the West Atlantic
was Beroe, another ctenophore. The subsequent appearance of this predator in the Black Sea
ecosystem remains unexplained, but seems a likely cause for a subsequent reduction in
Mnemiopsis stocks. A later modeling exercise with Russian colleagues provided a more detailed
explanation of these ecosystem changes in the pelagic food web. What seems evident in retrospect
is that overfishing and eutrophication led to a complex sequence of ecosystem impacts. These have
had a far-reaching and so far unchecked, impact on the fauna and flora of this inland sea.
EU16) ______ (2001) A brief overview of catchment basin effects on marine fisheries. (Chapter 18,
p 355-370 In: Coastal Hypoxia: consequences for living resources and ecosystems. (American
Geophysical Union).
EU15) De Leiva Moreno, J.I., V.N. Agostini, J.F. Caddy and F. Carocci. (2000). Is the pelagicdemersal ratio from fisheries landings a useful proxy for nutrient availability? A preliminary data
exploration for the semi-enclosed seas around Europe. ICES Journal Mar. Sci. 57: p 1091-1102.
EU14) ________ (2000). Letter to the editor of fisheries research: Ecosystems structure before
fishing? Fish. Res. 47; pp 101-102.
EU13) _______ (2000). Marine catchment basin effects versus impacts of fisheries on semienclosed seas. ICES J. Mar. Sci. 57: p 628-640.
EU12) Fiorentini, L. and J.F. Caddy and J.I. de Leiva (1997). Long- and short-term trends of
Mediterranean fishery resources. GFCM Stud. Rev., 69:72 p.
EU11) ________, R. Refk and T. Do Chi (1996). Productivity estimates for the Mediterranean:
Evidence for accelerating ecological change. Ocean and Coastal Management, 26(1):1-18
EU10)_________R. Refk and T. Do-Chi. (1995). Productivity estimates for the Mediterranean:
evidence of accelerating ecological change. P 1-18, In: FAO (1995). Effects of riverine inputs on
coastal ecosystems and fisheries resources. FAO Fish. Tech. Pap. 349; 132p.
EU9)________, and A. Bakun (1995). Marine catchment basins and anthropogenic effects on coastal
fishery ecosystems. P119-132 In: FAO (1995). Effects of riverine inputs on coastal ecosystems and
fisheries resources. FAO Fish. Tech. Pap. 349; 132p.
EU 8) Christensen V. and J. F. Caddy (1993). Reflections on the pelagic food web structure in the
Black Sea. (FAO Fish. Rep. No. 495 Annex IV, p 60-77; ICLARM Contribution 881).
EU7)________ (1993). Towards a comparative evaluation of human impacts on fishery ecosystems
of enclosed and semi-enclosed seas. Reviews in Fisheries Science 1(1): p 57-95.
EU6)________ (1993). A contrast between recent fishery trends and evidence for nutrient enrichment
in two large marine ecosystems: the Mediterranean and the Black Sea. (Chapter 14 in Large Marine
Ecosystems; AAAS Press; (p137-146 In: Proceedings of Symposium on Large Marine Ecosystems;
Monaco). (K. Sherman, L.M. Alexander and B.D. Gold eds).
EU5)_______ and A. Bakun. (1993). Marine catchment basins and anthropogenic effects on
coastal fishery ecosystems. Presented at: ‘2nd Int’l Conf. on environmental management of
enclosed coastal seas’ – Baltimore USA 10-13 Nov 1993.
EU4) ________ (1990). The protection of sensitive sea areas: a perspective on the conservation of
critical marine habitats of importance to marine fisheries. Int'l Seminar on the protection of Sensitive
Sea Areas: Malmo, Sweden: 25-28 Sept 1990,15p.
EU3)_______(1990): Mediterranean and Black Sea Fisheries: a contrast between recent fishery
trends in evidence for nutrient enrichment. Conference on Large Marine Ecosystems: Monaco,
October 1990.
EU2)_______(1979). The influence of variations in the seasonal temperature regime on
survival of larval stages of the American lobster (Homarus americanus) in the Southern Gulf
of St Lawrence. Rapp. proc. Verb, des Reun. Cons. Intl. Explor. Mer 175, p 204-216
EU1)______(1979). Long-term trends and evidence for production cycles in the Bay of Fundy
scallop fishery. Rapp. P.-v.. Reun. Cons. int. Explor. Mer, 175: p 97-108.