Date of Award

Spring 5-4-2017

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Marine Biology


Graham Sherwood

Second Committee Member

Lisa Kerr

Third Committee Member

Gayle Zydlewski


Atlantic cod (Gadus morhua), yellowtail flounder (Limanda ferruginea), and haddock (Melanogrammus aeglefinnus) were once dominant species in the New England fisheries economy, together accounting for over half of the landings value of groundfish. Over the last several decades, all three species have experienced dramatic shifts in spawning stock biomass (SSB) with current estimates for cod stocks at 3% and 7% of target biomass (Gulf of Maine and Georges Bank stocks, respectively), a strong contrast to haddock stocks that are nearly fully recovered (NEFSC 2014, 2017). As principally demersal species, they are easily targeted by trawl and gillnet, the former representing the majority of landings in New England waters. Now considered a choke species for the entire groundfish fleet, the current state of cod may signal the demise of other fish stocks if management measures are not adjusted. Despite a 2010 shift from input controls to a mixture of input controls (i.e., year-round closures) and output controls (i.e., hard quotas), along with yield-based stock assessment reference points, the outcomes for these key groundfish have been confounding. Some suggest that the failure to account for age structure of populations in management may underplay the value of old fish, those which are generally removed by commercial fishing (Le Bris 2013, Secor 2015, Stige et al. 2017). Age truncation, vulnerability to overoptimistic assessment, and increasing amplitude of climate oscillations all put the recovery of certain species at risk (Berkeley 2004, Pershing et al. 2015). One of the key management changes enacted with the Sustainable Fisheries Act of 1996, was the implementation of year-round closures throughout New England waters. Heralded as a backstop to uncertainty, these temporary Marine Protected Areas impart a conservative management approach with the simple goal of reducing overall mortality. Nearly 25 years after the first areas were closed to fishing, research has been inconclusive about the effects on the groundfish species that they were designed to protect (Murawski et al 2005, Kerr et al 2012). Until now, no study has investigated the effects of these closures on the age structure of these three species. To measure change, I applied three age metrics: mean age, age diversity, and catch-per-unit-effort of age 5+ fish. Together, these three metrics provide measures of change across large spatial and temporal scales.

In Chapter 1 of this thesis, I employ a Before-After Control-Impact (BACI) analysis to five year-round fisheries closures in New England. Data was drawn from over 35 years of the New England Fisheries Science Center’s bottom-trawl survey. Using stratified-random and distance-based controls, I employed generalized linear models (GLMs) and hurdle models to define significant changes to groundfish age structure. Further investigations describe the temporal response to closed areas and discuss the different ways that closure effects on age structure can manifest. Results show that in several cases, these closures function to improve age metrics in the species they were designed to protect. Discussion of the magnitude of change and confounding management factors lead to future considerations of spatial management of groundfish.

In Chapter 2 of this thesis, I focus on Cashes Ledge Closed Area (CL). Designed to reduce groundfish mortality, its effects on the age structure recovery on Atlantic cod are unclear. Previous work here has shown that in areas where older fish range, adjacent to Essential Fish Habitat (EFH), catch inside closures can produce up to 8 times more old cod (age 5+) than surrounding open areas (Sherwood & Grabowski 2016). While the benefits of protected areas assigned EFH status seem obvious, it is unclear how age structure of the cod population in this area responds across distance and varying habitat. Results on cod from Chapter 1 show limited sampling throughout the closure, particularly over complex bottom that is difficult to trawl. With a survey designed to measure the same age metrics as Chapter 1, I sampled sites using alternate gear types (gillnet and handline) across the entire closure and adjacent fishing grounds to the Northwest. Results showing a significant positive closure effect support those of previous work (Sherwood & Grabowski 2016), but the subtle magnitude of change and results of habitat modeling, reveal that nearly 15 years after the closure was implemented, depth is the major driver of these measured age structure metrics. Highest age structure metrics manifest over habitat characterized by shallow and complex bottom, which serve as havens for a species under serious threat from fishing mortality and climate change.

Chapter 1 provides a broad perspective of change over large scales of space and time. Samples are grouped over the entire closures and before-after periods include as many as 25 years of data. Chapter 2 provides a detailed investigation of a single closure over two summer/fall sampling seasons, 15 years after closure. Together these chapters provide perspective on what age structure health looks like and its lasting implications on the future of the groundfish fishery in New England.

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