Industry and conservation goals are complementary for the most valuable fishery in the United States under climate‐driven life history changes

Crustaceans, which are highly susceptible to the effects of climate change, are critical for food security worldwide. Yet, management rarely evaluates the performance of alternative regulatory strategies under climate‐driven life history change. This limits the development of climate‐ready management plans, undermining fisheries sustainability. We compared the performance of alternative minimum legal size (MLS) regulations under shifts in growth and maturity for American lobster in the Gulf of Maine, the most valuable single‐species commercial fishery in the United States. Across the life history change scenarios examined, increasing MLS improved status indicators, while decreasing MLS eroded status indicators for spawning stock biomass, legal abundance, landings, and exploitation rate. Our results demonstrate that protecting the lobster stock by increasing MLS improves fishery output, highlighting that conservation and industry goals can be complementary. This study exemplifies the utility of MLS as a conservation measure for crustacean fisheries under climate change.


INTRODUCTION
Fisheries are critically important for food security worldwide, with demand projected to double by the mid-century (Hollowed et al., 2013;Naylor et al., 2021).However, anthropogenic climate change is driving the warming of marine environments, which is expected to affect the productivity of fisheries (Blanchard et al., 2012).Fisheries are typically assessed and managed under the assumption of stationarity of underlying processes, especially life history characteristics (Szuwalski & Hollowed, 2016), though there are ongoing efforts to expand assessment and management techniques to account for nonstationary biological processes, such as in growth (Zhang et al., 2021) and mortality (Johnson et al., 2015).Managing fisheries assuming stationarity is problematic because it undermines their sustainability.Evaluating the performance of management actions under climate-driven shifts in life history characteristics is necessary for developing well-informed management plans.
Crustaceans, which are becoming an increasingly important contributor to global fisheries (Boenish et al., 2021), are particularly susceptible to warming-induced life history changes (Green et al., 2014).Specifically, crustaceans are expected to reach maturity at smaller sizes, molt more frequently, but grow less in size per molt (Green et al., 2014).These impacts on life history will influence stock dynamics because they change the size and timing at which individuals begin contributing to the spawning stock.Additionally, crustacean fisheries are commonly managed with the goal of protecting reproductive capacity (Botsford, 1991).Thus, if shifts in maturity and molting phenology are expected in the future, it behooves managers to evaluate the performance of regulations under changes to life history (Bell et al., 2020;Pinsky & Mantua, 2014).
American lobster (Homarus americanus) is the most valuable single-species fishery in the United States, and is the socioeconomic lifeblood for coastal communities in the Gulf of Maine (GOM) (ASMFC, 2020).Khalsa et al. (2023) found that shifts in growth and maturity are influential on the population and fishery dynamics of lobster, increasing the spawning stock biomass (SSB) and landings, albeit under status quo management.Despite lobster dynamics being sensitive to growth and maturity, there is a paucity of research that considers how these life history changes should be accounted for in management (but see Le Bris et al. (2018)).This is of particular concern given that the GOM is one of the fastest warming regions on Earth (Rheuban et al., 2017).
Management of the GOM American lobster fishery focuses on protecting the spawning stock through a combination of minimum and maximum legal sizes and v-notching, which have been found to have contributed to strong historical stock productivity (ASMFC, 2020;Le Bris et al., 2018;Li, 2018;Mazur et al., 2019).Thus far, only a handful of studies have evaluated the contribution of conservation measures to the productivity of the GOM lobster fishery.Alternative v-notching compliance rates and definitions were evaluated under varying recruitment levels, finding that the v-notching program was an instrumental driver of unprecedented historical productivity for the GOM lobster stock (Mazur et al., 2019).Le Bris et al. (2018) showed that protection of reproductive females through v-notching and maximum legal-size regulations, alongside environmental changes, contributed to improved lobster stock productivity in the GOM.They further demonstrated that applying the same management used in the GOM to the Southern New England stock, which is collapsed (ASMFC, 2020), would have mitigated some negative impacts of warming-induced stress.Finally, Li (2018) found that minimum legal size (MLS) had a larger impact than maximum legal size on the landings, abundance, and size composition of GOM lobster.
While these studies are useful for understanding the importance of conservation measures applied to the GOM lobster stock, they are limited because they do not incorporate anticipated future changes to growth and maturity alongside MLS alternatives.This is of paramount importance given the nature of the lobster fishery.As a recruitment fishery, over 90% of landings come from lobsters that molted into the legal size-class that year (Tanaka et al., 2019).Thus, changes to size-at-maturity (SAM) and molting phenology may have strong interactions with MLS, which is critical to understand for effective future management of the lobster fishery.
Here, we use a peer-reviewed individual-based lobster simulator (IBLS; Chang, 2015;Chen et al., 2005;Mazur et al., 2018) to expand upon the aforementioned studies and Khalsa et al. (2023) by simulating how status quo and alternative MLSs perform when lobster growth and maturity are shifted as anticipated under climate change.Not only can these findings help inform climate-ready lobster fishery management, but they can also be used to prioritize future research.Our overarching objective was to evaluate the performance of alternative MLS regulations under changes to SAM, molting probability, and molt increment size.

Individual-based lobster simulator
For simulations, we used the previously developed IBLS (Chang, 2015;Chen et al., 2005;Mazur et al., 2018), as described in Khalsa et al. (2023).The IBLS processes are explained in the supplementary material (IBLS Overview; Figure S1).Simulations used a weak stock-recruitment relationship, which links SSB (lagged by 6 years) to recruitment and best approximates the relationship over the simulation period.This is not a theoretical stock-recruitment relationship (e.g., Ricker or Beverton-Holt), but provides a link between spawning stock size and recruitment.In this relationship, recruitment is drawn from a normal distribution with means and standard deviations that are estimated from stock assessment outputs (ASMFC, 2015) for five levels of SSB.

Simulations
Simulation study design followed the same protocols as described in Khalsa et al. (2023).Using the IBLS, we

Analysis
For our analysis, we compared status over time among scenarios based on four indicators that are currently used for management: SSB, legal abundance, landings, and exploitation rate (ASMFC, 2020).For each indicator, the percentile-based reference point was the calculated 25th and 75th percentile for the time-series.We chose this approach, rather than calculating reference points over a historical reference period, as in the stock assessment (ASMFC, 2020), because we observed dramatic increases in SSB, abundance, and landings over time.Accounting for the strong positive trajectories of the stock dynamics in the reference point calculations resulted in more conservative status determinations.After calculating the reference points, the fishery was determined to be in either a negative, neutral, or positive state each year by comparing the indicator to the reference points (Table 2).Reference points and status determinations were conducted for every iteration of each scenario independently.

RESULTS
Due to the historical increase in the fishery, all indicators tended to be negative in the early years and transition to positive in the final years.However, this general trend varied under the different scenarios examined.

Spawning stock biomass
The SSB indicator status was negative across all scenarios from 1982 to 1989 (Figure 1).Between 1990 and 2005, the status was most often neutral under most scenarios.
After 2005, the status became positive across most scenarios.Climate-driven changes in growth and maturity as well as increasing MLS had a marked positive impact on the status.In contrast, decreasing MLS resulted in a lower number of iterations and years with a positive SSB indicator status and more with a neutral status.The least number of positive years and iterations were observed under the no change and small life history parameterizations paired with an MLS decrease of −10 mm; scenarios that had pronounced decreases in the SSB indicator status over the latter half of the time-series.

Legal abundance
The legal abundance indicator status improved over time across most scenarios (Figure 2).The indicator status was negative from 1982 to 1989, before switching to neutral between 1990 and 2004, after which point becoming mostly positive, with a few years with a neutral status.Improvements to the indicator status tracked increasingly great climate-driven shifts to growth and maturity.In general, increasing MLS improved the legal abundance indicator status, except for when the moderate life history change was combined with a +10 mm MLS alternative, and when the large life history change was paired with the +5 mm MLS parameterization.The poorest status was observed under no change or small changes to life history, combined with decreasing MLS.No change to life history paired with −5 and −10 mm MLS alternatives led to an improvement in indicator status from ∼1982 to 1995, followed by a sharp decline to mostly neutral or negative for the remainder of the time-series.This pattern was also observed under the small life history shift when MLS was reduced by 10 mm.

Landings
The general pattern of the landings indicator status was an increase from negative, to neutral, then positive over time, except for scenarios with the most extreme decreases to MLS (Figure 3).Under most combinations of life history and MLS parameterizations, the indicator status was mostly negative from 1982 to 1989, transitioning to neutral until ∼2000, then oscillating between neutral and positive for the remaining years, ultimately ending positive in the terminal year.The most pronounced differences were between scenarios that increased MLS versus those that decreased MLS.When MLS was decreased 1 mm or more with no change to life history, the indicator status degraded, with an increasing number of iterations having a neutral status.This pattern held for the small life history change scenario under a −10 mm MLS alternative.Further, terminal year status was neutral for most iterations, rather than positive for three scenarios: no change to life history paired with −5 and −10 mm MLS alternatives, and a small change to life history with a −10 mm MLS alternative.

Exploitation rate
The exploitation rate indicator status was variable among scenarios, but generally oscillated between negative and neutral in the 1980s, after which point it would become neutral or positive until the late-1990s to mid-2000s, before improving again to neutral or positive (Figure 4).Increasingly strong climate-driven shifts in life history improved The top and y-axis labels denote the life history (base case, small, moderate, and large) and MLS parameterizations, respectively.
with a majority of years having neutral or positive status prior to 2000, after which point the status switched to mostly neutral or negative.

DISCUSSION
Using the IBLS, we compared the performance of alternative MLSs for the GOM American lobster fishery when growth and maturity-related life history characteristics were shifted as expected under future warming.Status indicators improved under increasingly extreme shifts in SAM, molting probability, and molt increment size.Additionally, findings indicated an improvement of indicator status when MLS was increased, but a deterioration of indicator status when MLS was decreased.This pattern was especially pronounced when life history parameters underwent no change or a small change, coupled with decreases in MLS of 5 and 10 mm.These findings should be considered when developing regulation changes for the GOM American lobster fishery.MLS was increased in the late 1980s as a proactive move in response to consistently high fishery exploitation to allow more lobsters to mature and reproduce before being harvested, which preceded abrupt increases in landings into the 1990s (ASMFC, 2020).Protecting reproductive potential through legal size limits and v-notching regulations has been an integral contributor to productivity of the stock (Le Bris et al., 2018;Mazur et al., 2019).Our findings further indicate that preserving the reproductive potential of the stock by raising MLS may be possible without diminishing landings.In fact, increasing MLS within the range we tested led to an improvement in the landings and exploitation rate indicators simultaneously over time, demonstrating that landings could increase without risk of overfishing the population.The positive effect of raising MLS was most pronounced when life history was not changed substantially, suggesting that increasing MLS even under static life history characteristics would be advantageous.An important point to consider is that shifts in molt increment probability, molting probability, and SAM have inconsistent effects on lobster population and fishery dynamics (Khalsa et al., 2023).In this study, we only considered these life history changes in tandem, but in reality, these life history shifts will likely manifest with more variability, which could produce differing trajectories for the population and fishery dynamics (Khalsa et al., 2023).These findings are in agreement with Li (2018), who demonstrated that +1 and +2 mm changes to MLS increased both abundance and landings under static life history characteristics.
As a result of increasing MLS, the difference in size between when lobsters mature and are subject to harvest would increase, further protecting the spawning stock.Khalsa et al. (2023) found that under status quo management, SSB and landings increased substantially under the same small, moderate, and large scenarios utilized here.Increasing MLS would magnify the positive response of the spawning stock to growth and maturity-related shifts in life history, an important consideration for management under future climate change.While promising, these results should be interpreted thoughtfully.An artifact of the weak stock-recruitment relationship based on the 1982-2013 time-series is that there is a strong nonlinear positive relationship between SSB and recruitment (Li, 2018;Mazur et al., 2019).Thus, our results may be based on optimistic estimates of recruitment.Despite SSB increasing, if the spawning stock becomes dominated by smaller females, egg quality and quantity could diminish (Hixon et al., 2014;Koopman et al., 2015;Mazur et al., 2019;Ouellet & Plante, 2004).Furthermore, if temperatures enter a range that is physiologically stressful for lobster, as has already led to stock collapse in Southern New England (ASMFC, 2020;Quinn, 2017;Shields, 2013), the positive effects of climate change on SSB and landings may be dampened.A complement to our results is that Le Bris et al. (2018) found that conservation measures currently used in the GOM helped the stock benefit from historical warming and would have reduced the negative impacts of climate change on the Southern New England stock.Especially given these caveats, increasing MLS will likely be an important management action given expected climate change in the GOM.
While our study supports increasing MLS to bolster stock sustainability and productivity, careful considerations need to be made when setting regulations.Interactions between minimum and maximum legal sizes and v-notching-which could be explored using the current model framework-need to be pondered because when changed in tandem they may produce a mixed picture (Li, 2018).Under climate change, females reaching maturity at smaller sizes would become protected by v-notching sooner.If females became protected earlier, changing MLS may not be necessary to achieve the desired conservation goals.In fact, MLS may even be decreased while maintaining the spawning stock at a sustainable level.Since the conservation measures are primarily designed to protect females (Steneck et al., 2017), regulations that are too conservative may lead to a skewed sex ratio as fewer legal females are available to harvest.
Our findings are useful for informing the management of other crustacean fisheries.MLS is one of the oldest forms of fisheries regulations and is popular with managers because it is acceptable to the general public and relatively easy to understand and enforce (Stewart, 2008).Due to their relative ease of implementation, managers of emerging crustacean fisheries may consider setting MLSs in an attempt to protect the spawning stock and promote fishery sustainability without seriously reducing landings.However, managers should be cognizant of additional considerations relevant to their fishery, such as differ-ences in life history, stock dynamics, or other management measures that may influence how regulations should be implemented on a case-by-case basis.
Future research should be expanded to include the full suite of conservation measures to elucidate their interactive effects on the population and fishery dynamics.Additionally, life history changes that may occur but were not explored here, such as changing natural mortality and recruitment dynamics would be useful to examine in future simulation experiments.The findings of this study suggest that the conservation goal of preserving the spawning stock through MLS increases can be achieved while simultaneously bolstering fishery landings.In short, our findings demonstrate that conservation and harvest goals are complementary.On the whole, this study is an integral step in the direction of developing climate-ready management for the GOM American lobster fishery and highlights the importance of undergoing these research efforts for the ongoing sustainable management under unprecedented environmental change.

TA B L E 2
Indicator status determination scheme based on percentile-based reference points for each indicator.Status over time for the SSB indicator.Bar size indicates the number of iterations (n = 50) with a given status.The top and y-axis labels denote the life history (base case, small, moderate, and large) and MLS parameterizations, respectively.

F
I G U R E 2 Status over time for the legal abundance indicator.Bar size indicates the number of iterations (n = 50) with a given status.The top and y-axis labels denote the life history (base case, small, moderate, and large) and MLS parameterizations, respectively.F I G U R E 3 Status over time for the landings indicator.Bar size represents the number of iterations (n = 50) with a given status.The top and y-axis labels denote the life history (base case, small, moderate, and large) and MLS parameterizations, respectively.the status overall, especially in the latter half of the time-series across scenarios.Increasing MLS improved the status, increasing the number of neutral and positive years after 2000.In contrast, decreasing MLS, especially −5 and −10 mm reduced indicator status to mostly neutral or negative towards the end of the time-series.The −5 and −10 mm MLS alternatives also resulted in a reversal in the pattern of the exploitation rate indicator status improving over time, F I G U R E 4 Status over time for the exploitation rate indicator.Bar size represents the number of iterations (n = 50) with a given status.

A
C K N O W L E D G M E N T S Dr. Mackenzie Mazur, Dr. Conor McManus, Jesica Waller, Kathleen Reardon, Jeff Kipp, Dr. Tracy Pugh, and Dr. Sam Truesdell were instrumental in providing expert advice that informed the development of the simulation scenarios.This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. 1840992.Partial funding was provided from the NOAA Grant for American Lobster Research (NA21OAR4170371).D ATA AVA I L A B I L I T Y S TAT E M E N T Data will be made available upon reasonable request.O R C I D Noah Hunt https://orcid.org/0000-0002-2446-4884R E F E R E N C E S Parameterizations for the counterfactual simulations conducted using the IBLS.MLS, minimum legal size; MIP, molt increment probability; MP, molting probability; and SAM, size-at-maturity.+1 mm CL, +2 mm CL, +5 mm CL, +10 mm CL, −1 mm CL, −2 mm CL, −5 mm CL, and −10 mm CL TA B L E 1 and/or MLS over the 1982-2013 period.Small, moderate, and large directional changes to molt increment probability, molting probability, and size-at-maturity, which were informed based on the literature and an expert survey, simulated lobsters maturing at smaller sizes, growing less per molt, and molting less frequently; life history changes that are anticipated in a warming environment (ASMFC, 2020).