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Thread: Interesting sockeye study

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    Member fishNphysician's Avatar
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    Default Interesting sockeye study

    Unaccounted mortality in salmon fisheries: non-retention in gillnets and effects on estimates of spawners


    Matthew R. Baker*1 and Daniel E. Schindler 1,2
    1 School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, WA 98195-5020, USA ; and
    2 Department of Biology, University of Washington, Seattle, WA 98195-1800, USA

    *Correspondence author. E-mail: mattbakr@u.washington.edu


    Copyright Journal compilation © 2009 British Ecological Society


    KEYWORDS

    delayed mortality • ecosystem engineers • fishery-induced injury • mark–recapture analysis • natural resource management • Pacific salmon • population dynamics • stock-recruitment estimation


    ABSTRACT


    1. Effective and sustainable natural resource management is enhanced when the consequences of exploitative practices are fully understood and acknowledged. Commercial fisheries devote considerable resources to maximize the harvest of target species and minimize interference with non-target stocks. Appropriately, bycatch and discard of non-target stocks are recognized as critical economic and conservation concerns. Few studies, however, have examined non-retention mortality in target stocks. Non-retention, where fish are engaged by fishing gear but not landed, is rarely quantified and the effects on stocks are unknown. Mortality due to non-retention may have important effects on the dynamics of exploited populations.
    2. We surveyed spawning populations of sockeye salmon Oncorhynchus nerka that had traversed commercial fisheries in Bristol Bay, Alaska, to estimate the incidence of non-retention in gillnets and the severity of injuries associated with entanglement. To better understand how gillnet injury affects spawning success, we tagged and monitored stream-spawning fish and applied a maximum likelihood model to mark–recapture data.
    3. A substantial portion (11–29%) of spawning sockeye salmon exhibited clear signs of past entanglement with commercial gillnets. Survival among such fish was significantly reduced. More than half of the fish that reach natal spawning grounds with fishery-related injuries fail to reproduce. This suggests that estimates of spawning stocks are inflated by 5–15% at minimum.
    4. Synthesis and applications. Our analyses indicate that non-retention in gillnet fisheries is an important and under-appreciated consequence of the exploitation of salmon. Stock estimates for exploited populations that do not account for non-retention mortality overestimate the number of reproductively viable fish. Unaccounted mortality and interannual variation in the magnitude of this mortality may prevent accurate estimates of viable spawners, confound our understanding of the relationship between stock size and recruitment, impede optimal management and obscure the ecosystem impacts of migratory stocks in coastal watersheds. Given the magnitude of non-retention in this fishery, explicit consideration of non-retention mortality may be warranted across a wide range of exploited populations.


    Full article here....


    http://74.125.95.132/search?q=cache:...&ct=clnk&gl=us


    Some additional excerpts...


    "We estimated the incidence and severity of injuries in fish returning to natal streams and the effect of such injuries on pre-spawning mortality. The findings suggest that gillnet injuries are common and, in many cases, inhibit spawning."

    ****

    "It is clear that virtually all fish with moderate to severe gillnet injury fail to spawn. In the case of fish with minor injuries, the delay in stream entry, abbreviated stream residence and the inhibition of morphological traits associated with sexual maturation (fig. 1) suggest that even minor injuries delay maturation and reduce reproductive fitness."

    ****

    "Pre-spawning mortality was highly correlated with and was likely facilitated by fungal infection, caused by Saprolegnia spp., a facultative parasite common in freshwater ecosystems. Saprolegnia spp. causes tissue damage, loss of epithelial integrity and osmoregulatoryfailure. It is associated with damaged epidermal tissue suggesting fish with gillnet injuries are particularly susceptible to such infections. Of 43 fish with fungal infection at the time of our tagging, only one successfully spawned. Many injured fish without Saprolegnia spp. at tagging presumably developed infections subsequently. Due to the close correlation between fungal infection and pre-spawning mortality, Saprolegnia spp. is likely to be the proximate cause of pre-spawningmortality in gillnet injured fish."

    Must be why these guys didn't make it....
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  2. #2

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    Doc,

    Thanks for the information. While I don't think it is a surprise so much that the commercial fishery does have problems with, it is nice to have a quantitative measure as this may change how our biologists manage our fishery. But then again I suppose once the political engines of the comercial fisheries hears this, there exhaust notes will be playing in five fart harmony if it aint broke don't fix it......and there is some validity in that argument. Anyway, it is an interesting read and thanks for sharing.

  3. #3

    Default Good link

    Thanks for the link. This substantiates what biologists and fishermen have long suspected.

    I don't know about other parts of the state, but for many years fishery managers in Bristol Bay have been adjusting fishing periods to reflect not only the quantity of escapement, but also the quality of the fish that get by the nets. This is of special concern when the fisheries are restricted to special harvest areas: in-river Naknek, Wood River, Egegik, etc. In those conditions, gillnets are crammed so tightly into such small areas that "catch and drop-out" is deemed to be especially prevalent and harmful to escaping fish.

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    Member BlueMoose's Avatar
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    Default Thanks

    That is a great post and as always Doc great information. I wonder what the effects are on a place like the Kenai or Russain that have thousands of Reds with hooks in backs and open flesh wounds from the Twitch.

    One would think it would not be as bad based on the fact the fish are almost in the spawning stage. Very Interesting subject.

    Moose-O

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    It all depends on the timing and extent of the flesh injury. The earlier it happens and the bigger or more numerous the wounds, the more likely the fish is to succumb.

    Remember a spawning salmon is basically in decomposition mode. It has stopped feeding and most of its energy stores are being converted to sperm/eggs. It has greatly diminished reserve for tissue re-growth/healing and its immune system is also relatively compromised. As long as its still living carcass can hold out long enough to deposit the goods into the gravel, it's all good.

    But this study clearly shows that gillnet wounds can literally leave the fish wide open to overwhelming fungal infection. They're either dead before they spawn, or are so physically impaired that they are unable to spawn.

    You brought up a good point where the sport camp is also guilty of diminishing reproductive success of sockeye salmon. The flip-N-rip crowd unnecessarily wounds MANY fish. Fish hooked in the mouth are retained for harvest.... snagged fish are released back to the fungus-laden flows where the wounds become infected. One small wound, probably NOT a big deal. But if that same fish gets snagged and released multiple times over its in-river migration, odds for successful spawning diminish.

    There's another under-appreciated flip-N-rip injury..... belly-snagging a sockeye full thickness thru the abdominal wall. Pathogen-laced river water gains free access to the peritoneal cavity, potentially causing a deadly peritonitis. Moreover, if the fish is a hen, free water in the belly will render her eggs NON-viable. Even if she survives long enough to complete the act of spawning, she deposits only dead eggs into the gravel. Not only is her reproductive potential wasted, but every buck unfortunate enough to bed with her will have wasted his seed as well.

    Just more reasons why rippers suck!
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    And it's not just sockeye "dropouts" that suffer mortal gillnet wounds....
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    So the actual escapement value in bristol bay is off by less than the margin of error in the escapement value (we are good, but not that good).

    My question: So what?

    I wish they'd look at fish in mixed stocked fisheries that are released or escape instead of a fishery that is not on the brink of collapse. But I'm sure these folks wanted a good excuse to go to Alaska, isn't that what masters projects are for?
    I choose to fly fish, not because its easy, but because its hard.

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    Default perspective.

    Enjoyed the post doc. Here are some additional thoughts. The authors note that this may lead to a misunderstanding of stock/recruitment realtionships and for some systems that may be true. In others it may not. In the Kenai ADF&G did some sensitivity studies of the stock/recruitment model to errors in the sonar counter (same issue just a different cause). The studies indicated that a fairly large error was needed (in some cases approaching 50%) to change the escapement goal. Why this level of error. Because the total return data is based on harvest and escapement and thus more than 70% of the total return was being driven by harvest.

    So that leaves 30% to deal with the error. Now in the mortality paper you reference they indicate that 11-29% may die. That means only 3-9 percent of the total return estimate is in error. That level would not drive a stock/recruitment curve to change goals because the goals are fairly wide in the Kenai. This would not change the goals but would bias the spawner return ratio if used for other purposes.

    Now in a system that is recovering with no harvest on it the error would be 11-29% and that may drive a new approach.

    Just some perspective in how to look at these type of data. Again, this is worthwhile stuff and managers and research biologist should take note depending on the application of the data and sensitivity of the studies to this issue.

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    Quote Originally Posted by Nerka View Post
    So that leaves 30% to deal with the error. Now in the mortality paper you reference they indicate that 11-29% may die. That means only 3-9 percent of the total return estimate is in error. That level would not drive a stock/recruitment curve to change goals because the goals are fairly wide in the Kenai. This would not change the goals but would bias the spawner return ratio if used for other purposes.

    I believe our counts are suppose to be within 5% of the actual value in bristol bay leaving the total escapement (counting the error associated with our method of counting on each bank for 10 minutes per hour) error about plus or minus 10%
    I choose to fly fish, not because its easy, but because its hard.

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    Default doubt this figure

    Quote Originally Posted by ak_powder_monkey View Post
    I believe our counts are suppose to be within 5% of the actual value in bristol bay leaving the total escapement (counting the error associated with our method of counting on each bank for 10 minutes per hour) error about plus or minus 10%
    ADF&G may say that based on the relationship of 10 min counts to hour counts but do not forget bias. The person doing the counting introduces a second source of error which can be significant. I have not looked at the BB data for awhile so again just pointing out potential sources of error. To count within 5% of the true value is something rarely reached in fisheries research on salmon enumeration.

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    Member ak_powder_monkey's Avatar
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    Quote Originally Posted by Nerka View Post
    ADF&G may say that based on the relationship of 10 min counts to hour counts but do not forget bias. The person doing the counting introduces a second source of error which can be significant. I have not looked at the BB data for awhile so again just pointing out potential sources of error. To count within 5% of the true value is something rarely reached in fisheries research on salmon enumeration.

    Yup that is a source of error for sure, we try to minimize it by having the crew leader and the field camp coordinator do occasional paired counts with the other crew members to minimize that. We strive to be within plus/minus 5% the actual value, but who knows if we are (well aerial counts jive with tower counts pretty good)
    I choose to fly fish, not because its easy, but because its hard.

  12. #12

    Default gillnet efficiency

    I have often wondered what the efficiency of different types of gillnets are and if gear modifications could help reduce "mortly wounded excapment".
    I have a little experience with both drift and set net gillnet fishing but I`m no statitician. Drift gear hangs relitivily limp in the water with slack in the meshes and probaly entagle the fish better and allows them to roll into other meshes. Set gillnets in tidal current areas tend to hang pretty tight and billow in the current making it more difficult for a fish to entangle in other meshes. My guess is that set nets probably have a higher bounce off and squirt through rate than drift gear.
    In the set net fishery that I am most familiar with I would say the overall efficiency is less than 30% of the fish that see the net actually get caught. Of the fish that contact the net I think maybe 15% escape. These are just guesses I have never seen a study on this.

    I know that when someone talks about changing gear eficiency other people get very nervous. A lot of our salmon regulation are in place to reduce eficieny. length of nets, distance apart, depth, mesh size, and lots of other ways. Nobody wants the other guy to catch more.
    It always seemed to me that if I could get my net in the water where there was fish that I should be able to catch all that contact my net or at least one in every hole.
    If we could reduce bounce and squirt mortality with more eficient gear would it be a good deal all the way around or would it start a bunch of new allocation battles in the commericial sector. Maybe these mortally wounded are good in the spawning grounds. They give the bears and birds easier prey and leave more healty fish to spawn.
    I`m interesred to hear some ideas and sure would like to use more eficient gear.

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    Member ak_powder_monkey's Avatar
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    set net efficiency depends on how the net is hung, as well as mesh size, most of the gillnet scarred fish we see are the smaller females I've seen very very few large males with gillnet scars.
    I choose to fly fish, not because its easy, but because its hard.

  14. #14

    Default gill net scars

    Ak Pdr Monkey It stands to reason that the smaller females would squirt through and be scarred. Are females more valuable to spawning than males?

    Its pretty hard to find the perfect 1 size fits all mesh espesially when you have a mix of 4-5-6 year old sockeye. I have played around with hang in ratios from .75-1 to 3-1.
    What I learned is more web equals more tangles and work so I stay with the old standard 2-1.

    Here is some interesting reading.

    http://www.dfo-mpo.gc.ca/Library/196260.pdf

    http://www.unuftp.is/static/fellows/document/jorge03prf.pdf

    ScienceDirect - Fisheries Research : Catching efficiency and selectivity of gi

    gill net efficiency - Google Search

    What I`m thinking of is tromels and multi panels to increase efficiency. They must work or it would`nt be illegal I know its an allocation isuue for the next guy down the line but I want every fish that hits my net and if it would increase the number of spawning females in the escapment it could be good.

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    Quote Originally Posted by fishNphysician View Post
    And it's not just sockeye "dropouts" that suffer mortal gillnet wounds....

    Too true doc,

    On the Nushagak, there can be a lot of kings with obvious net scars, usually on their snouts....if the kings come in later than usual and face the brunt of the peak red season, that number can seem like a third of the fish you catch have this issue. It's dissappointing to see, but with the power of the BB fleet, it's a tough one to get an ear to it as long as the escapement comes in okay.

    Monkey,
    They studied an intact population mostly because there is a high rate of commercial fishing, a consistent and generally accurate count, almost no sport fishing to shade the numbers, and....they have campuses in the Wood River State Park to launch these surveys from to do it as intensively as it required. This kind of study would have never flown in an area of high sport use or any disturbed kind of area. BUT, the results are completely applicable to other populations that have any kind of serious gillnet fishery.

    Like it or not, the university of washington does a whole buttload of good science for Alaskan fisheries, and the guy who wrote the article could certainly someday be your boss....wouldn't that chap your bottom.

    I'm not giving them carte blanche on all they do, but they are looking at topics and issues that the state of AK would probably rather not touch....but can definitely use the info from.

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    Member TYNMON's Avatar
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    Post spawn mortality on Kuskokwim Weirs... Observed pigh number of chum pre-spawn mortality on the Tuluksak and Kwethluk weirs.. ADFG was ABSSOLUTELY not interested in this info, although the USFWS continued to colllect the data....

    One doesn't have to wonder why they were not interested in the data....

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