Estimates of Predator Consumption of the Northern Shrimp Pandalus borealis with Implications for Estimates of Population Biomass in the Gulf of Maine

Abstract

Globally, shrimp stocks are important both economically and ecologically, and this is true for the northern shrimp Pandalus borealis in the Gulf of Maine. Understanding and modeling of shrimp stock dynamics have often posed challenges, leading to the suggestion that inclusion of more multidisciplinary data may help to better elucidate shrimp stock dynamics. Thus, we explored the potential implications of including additional information for northern shrimp biomass estimates by examining fish food habits from the Gulf of Maine. We estimated the consumptive removals by 18 predators of northern shrimp. Summed across all predators, the consumptive removals for northern shrimp in recent years were on the order of 20,000–30,000 metric tons. These estimates are generally conservative based upon consumption model assumptions. Compared with survey indices, the trends in consumptive removals were out of phase in the 1990s but both trended upwards in the early 2000s. Compared with the base assessment model, consumptive removals were of the same order of magnitude, trended similarly, and were two to four times higher than estimates of northern shrimp biomass. Changing the natural mortality (M) parameter in the assessment model to higher levels (M from 0.25 to 0.50 or 0.60) reconciled the difference in estimates of biomass. Our objective was neither to fit a new assessment model nor to provide precise estimates of northern shrimp biomass; rather, we demonstrate that including food habits data as contextual information in such assessments is feasible and should lead to enhanced estimates of M. Improvements in obtaining relative estimates of biomass could then result from these enhanced estimates of M, particularly as it would allow for evaluating the tradeoffs between fishing and predation as sources of northern shrimp removals.

The importance of including predation in fish population models: Implications for biological reference points

“…For instance, a recent review of the estimated consumptive removals of northern shrimp, Pandalus borealis, indicated that a natural mortality rate of 0.6 is more likely than the current value of 0.25 (NEFSC, 2007;Link and Idoine, 2009). When the revised 0.6 value was used to calculate abundance and biomass, an increase of 4-5 times was observed for both biomass and abundance of this species, aligning more closely in magnitude to estimates derived from predatory consumption.…”

“…Yet we note that even without requisite food habits data and the ability to model consumption directly, there are other approaches to generically include predation mortality in assessments (see below). For example, M need not be a fixed parameter; in terms of process error, having a time invariant natural mortality for stocks can lead to erroneous projections and estimates, as seen in the shrimp example above (NEFSC, 2007;Link and Idoine, 2009). This process error is magnified when an assessment is using an age or stage-based model, as noted in numerous examples in the previous section as compared to using an age invariant natural mortality (Tables 1 and 2).…”

“…Usually, at least in most fisheries contexts, there are some set of surveys that can estimate the abundances or biomasses of both predators and prey in a given ecosystem, which can then be statistically linked (e.g., GLMs relating abundance to % BW consumed or to other consumption values; sensu Overholtz et al, 2008) or linked via process modeling (e.g., various functional response forms; sensu Moustahfid et al, 2010). Certainly using such an indirect approach may initially only provide contextual information in stock assessments, perhaps only informing suggested revisions to M or Z (e.g., the Pandalus example above; NEFSC, 2007;Link and Idoine, 2009). And certainly the sensitivities to the parameters and functional forms will need to be examined and presented in full sensitivity or risk analysis contexts (Punt and Butterworth, 1995;Peterman, 2004;Kinzey and Punt, 2009).…”

Ecosystem‐based fisheries management in the Northwest Atlantic

“…predation or M2) mortality. In the northwest Atlantic, MRMs have been developed predominately for forage stocks, including Atlantic herring (Clupea harengus), Atlantic mackerel (Scomber scombrus), longfin squid (Loligo pealei), butterfish (Peprilus triacanthus), and Northern shrimp (Pandalus borealis) (NEFSC 2007a,b;Overholtz and Link 2007;Link and Sosebee 2008;Overholtz et al 2008a;Tyrrell et al 2008;Gamble and Link 2009;Link and Idoine 2009;Moustahfid et al 2009a,b). These approaches have the potential to be controversial because they produce more conservative BRPs and explicitly address the potential for competition between predators and fleets that target these stocks.…”

Phenological shifts in hatch timing of northern shrimp Pandalus borealis

“…The northern shrimp is an important food source for commercially important fish species (Link & Idoine 2009), and is itself the target of commercial fisheries throughout its circumboreal distribution in the North Atlantic Ocean (Bergström 2000). Inter-population comparisons and ABSTRACT: The northern shrimp Pandalus borealis reaches the southern limit of its distribution in the Gulf of Maine, USA.…”