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Subject: Deep Threats for High Seas
From: "Trevor J. Kenchington" <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Wed, 9 Aug 2006 21:36:55 -0300
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> Mike,
>
>> ==============================================
>> Trevor Kenchington wrote:
>>
>> However, your argument as stated was not confined to such  
>> environments and
>> was equally applicable to areas long trawled, where if there was  
>> ever a
>> stage of chewing up, it is
>> past and gone.
>> ==============================================
>>
>> Trevor, I often wonder if that past "chewing up" of high quality  
>> habitat is
>> why so many of our fisheries remain a shadow of their former self  
>> and may
>> never fully recover.  Would it be unreasnable to think that  
>> decades of
>> dragging may have already caused at least some irreparable harm to  
>> some
>> very important areas of our oceans (shallow or deep)?
>
> Absolutely not. (As in it would absolutely not be unreasonable to  
> think that.) The question can only be: Was the irreparable harm  
> caused by decades of dragging confined to a very few, very small,  
> very special areas or did it affect broad swaths of the fishing  
> grounds?
>
> The only well-documented case that I am immediately aware of is the  
> elimination of the bryozoan/oyster reefs in Foveaux Strait, New  
> Zealand which I noted this morning. However, I have little doubt  
> that other areas of molluscan reefs, coral forest and the like were  
> very seriously impacted by early trawling and dragging -- either by  
> just a few passes of gear or else by prolonged "nibbling" away over  
> decades.
>
> Whether the broad tops of Georges Bank or the Dogger once had a  
> rich epibenthic biota that was removed by trawling is a lot harder  
> to know. (I doubt that the endless miles of the Bering or Barents  
> seas did because trawling there started late enough that I would  
> expect some record of extensive catches of benthos would have  
> survived.) The Dogger was trawled from the 1840s, when marine  
> biology was barely getting started in the inter-tidal, so a rich  
> epibiota there could have been lost before any academics became  
> conscious of sublittoral benthos. However, trawling didn't start in  
> offshore waters of New England until 1905 and the first ever field  
> study of the seabed-impacts affair was Alexander's in 1912-13. He  
> concluded that there was no serious disturbance of the seabed and  
> he wouldn't have reached that point if the trawlers were dragging  
> up any amount of "trash". [The observed trip that took the most  
> benthic material averaged under 5 bushels per tow. That was in the  
> Great South Channel.]
>
> I guess I'm inclined to think that major environmental change  
> caused by the seabed impacts of trawls and dredges has been  
> confined to small patches and thus has not enormously reduced the  
> productivity of the fishery resources. But that is a very, very  
> tentative conclusion that may turn out to be very wrong.
>
>
>
>> For some context, consider the following...
>>
>>
>> ==============================================
>> From the chapter, "Ogunquit", in the book "A Song for the Blue  
>> Ocean" by
>> Carl
>> Safina (retyped by hand so sorry for any typos):
>>
>> Aboard the Bunny Clark today, Tim Tower does not need to read  
>> about the
>> 1500s, nor does he need his university diploma, to know what has  
>> been lost.
>>
>> "We used to catch big sea pollock, twenty to forty pounds, along  
>> the shore.
>> You never see that anymore; never see that," he says shaking his  
>> head and
>> looking at his shoes.  Sad and angry at the same time, he sees the  
>> basis of
>> his livelyhood being chipped away and being taken apart, fish by  
>> fish.
>>
>> And rock by rock. "These trawlers' net-dragging gear is destroying  
>> the
>> rocky bottom structure and the growth on it that the fish need as  
>> habitat,"
>> Tim says.  "Give you an example: There used to be a great pollock  
>> spot on
>> the back side of Jeffreys Ledge.  There was a hell of a school of  
>> pollock
>> there.  One other person knew about it, and he knew a dragger  
>> captain who
>> was having hard times, and he took some pity on him, and he gave  
>> him the
>> loran numbers.
>>
>> The next morning the dragger was working that peak.  The first  
>> tow, he got
>> eighty thousand pounds of pollock.  We never caught a significant  
>> amount of
>> pollock on that spot again.  And I'll tell you something; after that
>> dragger left, the entire bottom habitat was changed.  What used to  
>> be a
>> sharp, jagged peak on the sonar was a round, flattened hump.  In the
>> northeast corner there used to be a real sharp edge and all these  
>> little
>> anemones and stuff used to grow there.  Now it's nothing like it  
>> used to
>> be.  You can still catch pollock there occasionally, but - "   
>> Tower ends
>> his sentence with a dejected wave of his hands.
>> ==============================================
>
> I had forgotten that bit of Safina's book.
>
> As I may have explained in some previous posting to this list, in  
> 1990-91, I spent 6 months interviewing Nova Scotian hook-and-line  
> groundfish fishermen for Canada's DFO. It was an educational  
> experience in many ways but one of the lessons was that I got the  
> same story, up and down the coast, about the effects that draggers  
> had on the seabed, turning rocky ridges into mud. At first, I  
> dismissed that as the same sort of self-interested pleading that  
> fixed-gear fishermen had offered for centuries (including  
> handliners objecting to the evils of longlining, when that was new  
> technology). However, I kept getting the same story from people who  
> knew nothing about the gear used 20 miles up the coast nor other  
> practical aspects of the fishery even that close -- suggesting that  
> the consistency of the story about dragging wasn't a matter of the  
> same tale getting communicated to and fro.
>
> Eventually, I had to come up with some sort of hypothesis to  
> explain the longline fishermen's observations, since I could no  
> longer merely dismiss them. That hypothesis has been taken up and  
> published by others (without acknowledgement of its source!) but  
> there really isn't any evidence for it. It just remains one way to  
> explain what the longline guys saw. For what it is worth:
>
> As the ice sheet retreated 10,000+ years ago, it dumped moraines  
> which, in typical fashion for deposition by ice, were an unsorted  
> mix of everything from rock flour to huge boulders. Also typical of  
> moraines, it had a hummocky surface (which looks like ridges in the  
> 2D view of an echo sounder). Shortly after, the sealevel rose  
> across the moraines and while the intertidal lay over them, wave  
> action washed the surface clear of fine sediment, leaving a layer  
> of clean cobbles and scattered boulders. However, sealevel rose  
> quickly and that lag deposit was only a thin "skin" about one  
> cobble deep. The resulting seabed is known to the geologists (who  
> do have evidence for thus much of the tale) as "Scotian Shelf  
> Drift" and it occurs in an irregular band along the coast at a  
> specific depth range.
>
> It is those cobbles (not bedrock) that the longline men called  
> "rocky". Their gear sometimes snags an epibenthic animal and brings  
> up the attached cobble, so they got to see what the stones were  
> like. It made for a very good fishing ground because cod much  
> prefer to take a baited hook that is lying on clean stones than one  
> lying on mud. (I don't know why. Somebody should ask a codfish.)  
> Whether that special sort of bottom was actually more productive  
> for cod or other resource species is hard to say. Whether it was so  
> much more productive as to outweigh its small areal extent, making  
> the Scotian Shelf Drift bottom the major generator of cod  
> production is even harder to know. But it was the best bottom for  
> the longline guys to set their gear.
>
> Canada made an effort to import British steam-trawler technology  
> before 1914 but it didn't go far. In the mid-1920s there was an  
> inquiry which heard all the now-familiar pro and con arguments and  
> in the end decided to keep the trawl fleet at just three vessels.  
> Some say that the fish merchants had invested heavily in dory  
> schooners and didn't want to either find capital for trawlers or  
> compete with those who could. It seems more likely that such  
> arguments swayed government than any concern over the benthos.  
> Nothing much changed until the Nova Scotia government had a new  
> report written in 1944. In the closing days of a war won by  
> technology and gasoline, it is not surprising that the  
> recommendation was to develop trawling. Still, that development was  
> rather localized and most small harbours remained with only fixed- 
> gear groundfish operations. The great bulk of the trawling from  
> 1945 (and even more 1960) to 1976 was by foreign vessels which  
> worked the banks, where large catches could be taken by large  
> vessels with big nets.
>
> In 1977, Canada took control of the fishery, used scientific  
> assessments which set TACs for each management unit (running from  
> the banks in to the coast), and encouraged the mid-sized dragger  
> fleet  (typically boats 65ft long) to expand to replace the foreign  
> effort. They (and others) were given quotas that added up to the  
> TACs, which were themselves based on passed landings by the  
> foreigners (as the major input data to the assessments). The Nova  
> Scotian dragger guys, however, did not go out to the banks (or did  
> not only do that). They worked their way up the shore, harvesting  
> quotas that perhaps could have been sustained on the banks by  
> fishing out every little hole and spot they could find.
>
> In the process of dragging over the hummocks, every now and then an  
> otter board would catch a cobble the wrong way and knock it clear  
> of the seabed (which we know can happen). That isn't a problem when  
> it happens once but when the same line is dragged a hundred times,  
> the lag deposit of clean cobbles can get broken up. The mixed  
> glacial sediment underneath is then not exposed to wave action (as  
> it was so long ago when sealevel was rising) and so is not washed  
> clean. Instead, the tide runs over the hummocks and through the  
> breaks in the cobble, eroding and spreading the mud fraction of the  
> old moraine, flattening the hummocks and coating everything in mud.  
> [That last bit is the really hypothetical stuff. Somebody needs to  
> look at that from a perspective of an understanding of erosion and  
> deposition that I don't have.]
>
>
> My first point is that at least one plausible explanation exists  
> for the hook-and-line fishermen's observations, so they cannot be  
> merely dismissed -- which claims about turning rock into mud with a  
> trawl so easily could be. However, my second is that at  least this  
> one explanation is specific to a rather particular kind of seabed  
> that exists in a narrow band off Nova Scotia and likely along the  
> coast of Maine too. There doesn't seem to be much reason to  
> extrapolate the area-specific observation to broader areas (even  
> though many fishermen who made the observation do want to make that  
> extrapolation). My third point is that the resulting habitat change  
> affected the catchability of cod on hooked gear, reducing the  
> productivity of the grounds as seen by hook-and-line fishermen.  
> Whether it also reduce the biological productivity of the seabed  
> for cod or other species is a much more open question. Whether such  
> reduction in fish production (if any) was great enough to  
> materially affect the total production of the resource must be even  
> more uncertain.
>
> But uncertain means uncertain, not improbable.
>
>
>
> Trevor Kenchington
>
>
>

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