Thursday, July 2, 2020

STRIPED BASS: MASSACHUSETTS STUDY COULD LEAD TO NEW RELEASE MORTALITY RATE


For a very long time, fishery mangers have assumed that 9 percent of all striped bass caught by anglers won’t survive release.  That figure comes from a 2011 study titled “Mortality of Striped Bass Hooked and Released in Salt Water.”

The authors of that study, Paul Diodati and Anne Richards, found that

“Depth of hook penetration in the oral cavity, anatomical site of hooking, gear type (single or treble hooks), and angler experience were significantly related to mortality…The final model included depth of hook penetration, gear type, and angler experience as predictor variables.  Predicted mortality ranged from 3% under the most favorable conditions to 26% for the worst set of conditions.  Predicted as well as observed mortality for the entire experiment was 9%...At the end of the experiment, condition factors were significantly lower for surviving hooked fish than for fish that had not been hooked.”
Many anglers have never been completely comfortable with the conclusions of the Diodati study.  Some, particularly including fly fishermen and others who do most of their fishing with single-hooked artificial lures, observe that, with a little reviving, all of their fish eventually swim away, and argue that their release mortality rate is well under 9 percent.

At the same time they, and others who catch their bass with cast lures, often point to those who “snag and drop” with live menhaden, chum their bass in with clams, or fish chunks of bait on the bottom, claiming that such activities lead to levels of release mortality well above the 9 percent standard.  Such people often also look askance at trolling for stripers, and claim that hooking a bass on a wire-line outfit, and dragging the fish through the water for a few hundred yards while it is being reeled in (because wire line boats don’t stop when a fish is hooked) also ups mortality numbers.

It’s possible that all those claims are right, which is why 9 percent represents a blended mortality rate that considers both the best and worst set of conditions that might confront a hooked fish.

It’s also possible that, under today’s conditions, the 9 percent number is no longer valid, and that a new estimate of release mortality is needed.



Unlike the 2011 study, the new Massachusetts study will not involve fish contained in a dammed salt pond.  Instead, the fish will be fitted with acoustic tags and returned to the waters of Salem Sound, on the northeast Massachusetts coast.  For the first half of the study, which will take place this year, 175 striped bass will be caught on natural bait, both dead and alive, which will be fished on both circle hooks and traditional J hooks.  Next year, the study will be expanded to examine survival rates with artificial lures, and the difference in survival rates between fish caught on single and treble hooks.

The goal of the study is to determine the number of fish that survive for at least two weeks after release.  The acoustic tags will record the swimming movements of the fish; cessation of such swimming motion will lead to a conclusion that the fish has died.  Massachusetts has set out 29 acoustic receivers, covering most of the water in Salem Sound, to receive the tags’ signals.  Should a fish leave Salem Sound for other waters, acoustic receivers elsewhere in state waters, and even in the waters of other states, can pick up its tag’s transmissions.

It’s anyone’s guess as to what the new study will find.

There’s undoubtedly a widespread assumption that fish hooked on circle hooks, which generally hook bass in the jaw and rarely result in gut-hooked fish, will show a significantly lower mortality rate than those hooked on J hooks, and that mortality for circle-hooked fish will be substantially below the current 9 percent standard.

But anyone who expects the overall mortality rate to be less than 9 percent might be jumping the gun.

The new Massachusetts study will differ from the 2011 study in a number of ways that could impact the results.  One is the size of the fish involved.

The striped bass observed in the 2011 study were small; their length ranged between 27 and 55 centimeters (roughly between 11 and 22 inches).  More fish fell into the 32-33 cm (about 13 inch) size range than any other.  

Compared to the bass typically encountered by recreational fishermen in the northeast, those are pretty tiny fish.  If the new study encounters larger bass than those included in the 2011 study, and particularly if some of those bass are substantially larger, the mere stress involved in capturing the bigger bass could lead to a higher release mortality rate that offsets the mortality savings of circle hook use to a greater or lesser degree.

The bass in the new study are also going to be released back into Salem Sound, rather than into a closed-off salt pond.  

That could impact release mortality in a couple of ways.

First, it could prove a direct benefit to the fish, as they could seek out water temperatures most conducive to their survival.  In the salt pond used for the 2011 study, water temperatures ranged between 15 and 28 degrees Centigrade (59 to 82 degrees Fahrenheit), with bottom temperatures rising as high as 25o C (77o F).  Temperatures at the high end of that range could definitely have had a detrimental impact on the released fish’s survival; if so, fish released into the presumably cooler waters of Salem Sound may not succumb to temperature-related mortality in as high numbers.

At the same time, the dammed-off salt pond offered the released fish some protection from marine predators.  The authors of the 2011 study reported that

“The condition factors of control fish held in the net-pen did not change significantly during the experiment, but conditions factors of hooked fish held in the net-pen were significantly lower at the end.  For striped bass recovered from the pond at the end of the experiment, condition factors of both hooked and not-hooked fish had decreased significantly from the start of the experiment, but condition factors of hooked fish were significantly lower than those of fish not hooked.  [references deleted]”
It doesn’t take much imagination to believe that hooked bass, released into an inshore ocean that supports healthy populations of seals, dolphin, and various sharks, at a time when they are not in as good condition as bass that have not been caught and released, will be both more attractive and more vulnerable to such predators, and so will die at a higher rate than fish contained within a closed-off salt pond.

Thus, it is hard to predict just how the new study will turn out, and whether average striped bass mortality will be found to be higher or lower than the currently accepted 9 percent.

And whatever mortality rate the study ultimately reveals, there is no guarantee that such stjudy will reflect what goes on in the real world.  As the 2011 study points out,

“Hooking mortality estimated from an experiment cannot predict population mortality unless experimental conditions represent those encountered in the wild.  For species with a broad geographic range, such as anadromous Atlantic striped bass, it seems unlikely that mortality recorded from an experiment would equal that of the population over a time period of interest (typically a year).  However, experiments can identify critical factors influencing hooking mortality and can be used to develop models that predict mortality, given values of those critical parameters…Our present model would not be sufficient for estimating coastwide hooking mortality of striped bass, as it does not include effects of such factors as fish size and environmental variables (temperature, salinity) on mortality.”
The 2011 study—and, from what I can tell, the Massachusetts study—also doesn’t include the effects of hanging a big bass from a Boga-Grip or Chatillon scale, waving it around in the air while someone digs around looking for a phone or a camera, tossing it back into the water from an elevated jetty, rock or party boat deck, dragging it onto and over the stones and the sand, or…

Well, you get the idea.

The results of the new study won’t be perfect.

But depending on the size of the fish caught, how they are handled and a number of other factors, the new study should provide a good idea of whether the 9 percent mortality rate is a good reflection of reality, or whether managers ought to use a lower—or higher—rate to better reflect the impacts of recreational release.

Either way, better information can only lead to better striped bass management.

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