Methods for estimating discard survival in fisheries: an integrated approach

Discarding Pacific halibut, FAO

ICES has published a report on methods for estimating discard survival in fisheries. The report details the results of the February, 2014 ICES WKMEDS workshop on discard survival.

“This report will:

-  describe the concepts behind assessing discard survival (Sections 2 and 3);

-  describe three different approaches for estimating survival (vitality assessment, captive observation and tagging) (Sections 4, 5 and 6); and 

-  provide guidance on the selection of the most appropriate approaches and experimental designs, as well as how to integrate and utilize information from them, with respect to specific discard survival objectives (Sections 3, 7, 8 and 9). 

Later versions of this report will cover in more detail: 

-  techniques for assessing survival using tagging and biotelemetry; and 

-  the most appropriate methods for analyzing and reporting survival data. 

It is assumed that the user of these guidance notes has sufficient scientific training, or at least access to suitable scientific support, to be able to conduct the techniques described in these notes in an appropriately systematic and disciplined manner. However, these guidance notes are intended also to be informative for other stakeholders associated with fishing (primarily fishers and managers) who wish to support and understand discard survival estimates.”

The ICES WKMEDS report is a summary of an integrated approach for estimating discard survival. The approach uses various combinations of vitality assessment, captive observation, and tagging to achieve realistic estimates for discard survival in fisheries. The combinations of methods are determined by scientists, stakeholders, and managers using evaluation and prioritization:

“the choice of which species in which fisheries to study depends upon several criteria: existing survival information, the biological traits of the species, its population status, magnitude of discarding, fishery characteristics, environmental characteristics, socio-economic value of the fishery, available resources, and management policy. The process of prioritizing is unlikely to be simple and may involve a number of iterations, where results of preliminary studies inform the final choice.”

The ICES WKMEDS report represents a new approach for estimating discard survival. Sources of information about objectives, priorities, resource implications, and time frames are included in a decision matrix. Managers can use the matrix to make informed choices about discarding in key fisheries and management units and what methods can be used for further study of discard survival. Initial calibration of vitality assessment using delayed mortality observations of discards creates validated indicators for survival. Then use of validated vitality assessment indicators such as RAMP (Reflex Action Mortality Predictors) can provide rapid real-time assessment of potential discard mortality on-board fishing vessels.

ICES. 2014. Report of the Workshop on Methods for Estimating Discard Survival (WKMEDS), 17–21 February 2014, ICES HQ, Copenhagen, Denmark. ICES CM 2014/ACOM:51. 114 pp.

Cautionary tale of rockfish barotrauma and survival: looks can be deceiving

 Yelloweye rockfish, ADFG

Canary rockfish, WDFW

Canary and yelloweye rockfish were captured by Hannah et al. 2014 at 46-174 m depth, retrieved to the surface, and then submerged to depth in specialized sea cages for evaluation of survival.

The authors state, “The external physical signs associated with extreme expansion and retention of swimbladder gas (pronounced barotrauma), including esophageal eversion, exophthalmia and ocular emphysema, were common for both species at these capture depths and were more frequent than in prior studies conducted at shallower depths. Despite similar frequencies of most external barotrauma signs, 48-h post-recompression survival of the two species diverged markedly as capture depth increased. Survival of yelloweye rockfish was above 80% across all capture depths, while survival of canary rockfish was lower, declining sharply to just 25% at capture depths greater than 135 m. Fish of both species that were alive after 48 h of caging displayed very few of the external signs of pronounced barotrauma and had a high submergence success rate when released at the surface.”

Survival and submergence success of canary and yelloweye rockfish, Hannah et al. 2014

Difficulty for evaluating vitality and potential survival by observing barotrauma symptoms and reflex actions is outlined by the authors. “The divergence of 48-h post-recompression survival of canary and yelloweye rockfish as depth of capture increased beyond 135 m shows how difficult it can be to evaluate the survival potential of rockfish with barotrauma based on their appearance at the surface. Most specimens of both species captured at these depths showed some signs of pronounced barotrauma, yet nearly all of the yelloweye rockfish survived following recompression while many of the canary rockfish perished as capture depth increased beyond about 75 m. Studies of post-recompression release behavior also support the notion that surface observations are not indicative of survival, at least for rockfish that tend to retain most of their expanded swimbladder gas (Hannah and Matteson, 2007; Hannah et al., 2008a). The retained gas can make it very difficult or impossible for rockfish to submerge (Hannah et al., 2008b; Hochhalter, 2012) and interferes with the evaluation of reflex behaviors, which have been shown to be useful predictors of survival in other captured and discarded fishes (Davis, 2007; Davis and Ottmar, 2006).”

With regards to stock management, the authors state, “The estimates developed in this study can be very useful for informing the management of hook-and-line fisheries that encounter these two overfished species, especially in combination with data on submergence success as a function of capture depth, like that provided by Hochhalter (2012) for yelloweye rockfish. For example, a primary recommendation from prior studies of post-recompression survival and submergence success for these two species was that hook-and-line fishers should use a variety of “descending” devices to help released fish overcome surface buoyancy (Theberge and Parker, 2005; Hochhalter and Reed, 2011; Hannah et al., 2012; Hochhalter, 2012). The data from this study suggest that descending devices may have a positive effect on survival of yelloweye rockfish across a wide depth range (Fig. 6, lower panel). However, for canary rockfish captured at depths greater than 135 m, survival may be so low that it might be better to either allow retention of these fish or to simply not allow a fishery to operate at these deeper depths (Fig. 6, upper panel).”