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Subject: Ecosystem management - any ideas?
From: Doug Cross <[log in to unmask]>
Reply-To:Academic forum on fisheries ecology and related topics <[log in to unmask]>
Date:Tue, 16 Jul 1996 11:35:16 +0000
Content-Type:TEXT/PLAIN
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Jennifer Upchurch asked on 9th July about information on the emergence of ecosystem management in fishery legislation. It seems that my experience may be of wider interest to the Group, so I have copied my reply to her here. Much of my work relating to this has only appeared in obscure publications or consultancy reports - not the best way to spread ideas, so I have included a couple of original sources in case anyone wants to chase them up.
 
 
Over the past few years I have worked on the dynamics of floodplain fisheries in Bangladesh and Pakistan (Jamuna and Indus Rivers). There is a major conflict of interest between agriculture and river fishery stock management, and agriculture almost always comes out on top. My concern has been that fish stock management proposals were being made which failed to relate to the actual dynamics of stock recruitment - in these rivers the main commercial crop - the major carps - have spawning characteristics which effectively make them similar to eels, in that they spawn at locations which are very remote from the actual fisheries. Consequently, recruitment is governed by both distant and local factors, and none of these were being taken into account by the fishery legislators.
 
On the Jamuna floodplain particularly, the people capture fish from their fields as the annual floods recede - this is known as the `receding flood fishery'. The assumption has always been that if flood protection bunds are erected to protect parts of the floodplain from flooding, then the consequent loss of fish to this fishery is directly proportionate to the area lost - i.e., if half of an area is partitioned off, then half of the fish production will go as well.
 
In fact, this is rubbish. I found that there is virtually no primary production in the main rivers during much of the year - it's too silty, and light penetration can be as little as a few centimetres. So the river itself is NOT a major energy capture system as far as the fish stocks are concerned. It is the floodplain itself that supplies most of the energy to maintain the fish stocks.
 
The major carp fry are only present in the floodwaters for a few days every two or four weeks (it's a lunar spawning cycle) for a couple of months. They move out onto the floodplain, and collect energy from the dry-season residues - plants, insects, and so on - which they convert to new protein and fats during the growing period on the flooded plain. When the flood recedes, they move back to the river, taking with them this stored energy as their substantially enlarged bodies. This serves to support both these fish and the carnivorous species which prey on them during the dry season when flows fall back into the main river channel. During the dry season, there may be some primary productivity to help maintain the fish stocks, but it is still quite low if, as in these cases, there remains a substantial silt load in the river.
 
The people have two ways of exploiting the river fish which are associated with the change in energy availability associated with the annual flooding cycle. The traditional way is to collect the grown (0+ and surviving 1+ and older) fish as the flood recedes - this means that they harvest energy collected by these fish, and which represents transformed residual energy which was on the floodplain at the end of the last growing season. This process can be maintained over a wide range of catch rates, and is dynamic, not static.
 
In recent years the introduction of aquaculture has encouraged people to catch the fry at the start of the flooding season. If this is efficiently done, then there will be fewer fry available to do the energy harvesting from the flooded plain. And even submersible floodbanks, put in to delay flooding of rice fields for a few weeks to allow rice to ripen, may obstruct the recruitment of these little `energy harvesters, so that regardless of the proportion of land `protected' the effect on recruitment and therefore productivity for the coming flood season may be dramatically reduced.
 
This approach to the energetic dynamics of the fishery is applicable to virtually all of the world's great floodplain rivers, and I believe that the diversity of the floodplain energy resource residuals probably accounts for the great biological diversity of the fish communities in such rivers, where the habitat range and food supplies appear to be totally unsuited to the development of such remarkable biodiversity.
 
In Pakistan, the fishery in Lake Manchar (the largest natural freshwater lake on the Indus system) was permanently wrecked around 1962 by the development of the barrages used for irrigation. The Sindh Province barrage at Sukkur, and also those upstream in the Punjab tributaries, progressively delayed the flood period until the floodwater only got into the overflow channel to Lake Manchar AFTER the end of the fry-transportation season in the lower Indus (early May to early June). And although the main species affected by this were the major carps, the decline in energy transfer and utilisation efficiency was so severe that all fish species populations crashed in the Lake - the whole energy basis of the Lake fish community was pushed below a level which could maintain the stocks as a whole (Cross D. in `Right Bank Master Plan, Working Paper No.47 - Fisheries Development'. Mott MacDonald International, Cambridge, UK, 1990; section 6.3 - Basic biology of River Indus Species, pp 40-49).
 
So, energetics is the key to understanding how floodplain fisheries work - and how legislation needs to be tailored to this natural process. I have published a couple of accounts of these fisheries dynamics, and even linked the Bangladesh system to the thermodynamics of systems far from equilibrium. For example, the energy transfer system is indeed highly dynamic, but if pushed too far it may reach a bifurcation point, beyond which the system falls to a lower energy level  which is irreversible. At this point, some species may be lost if energy availability within the river system falls too much, and they may never return naturally, even if the energy constraint is eased. (`Synchronicity, energetics, and the dynamics of the floodplain fisheries.' (Cross D, 1992). Proceedings of the 8th National Symposium of the Zoological Society of Bangladesh, Rajshahi University, Bangladesh, January 1992)
 
Unfortunately, to make the information available to the local fishery people, who have precious little access to scientific literature, these reports were published in very obscure locations! However, my point here is that fisheries should not be lumbered with clumsy baggage of steady state concepts (including that of the `climax community') and fixed biomass productivity. Describing population dynamics in numerical terms is not useful when energetics, chance and change play such important parts in regulating how stocks respond to natural and unnatural pressures on them.
 
 
I hope that these examples are of interest!
 
 
--Doug Cross, Ecologist --

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