{"id":4238,"date":"2026-01-09T13:28:26","date_gmt":"2026-01-09T18:28:26","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/ewemodel\/?post_type=chapter&p=4238"},"modified":"2026-04-07T18:29:43","modified_gmt":"2026-04-07T22:29:43","slug":"marine-mammals-and-fisheries","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/ewemodel\/chapter\/marine-mammals-and-fisheries\/","title":{"raw":"Marine mammals and fisheries","rendered":"Marine mammals and fisheries"},"content":{"raw":"

What are the ecosystem-level impacts and economic consequences of prioritizing marine mammal recovery versus fisheries yields, and how can management strategies be optimized to balance these competing objectives?<\/h2>\r\n
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This overarching question can be explored using EwE to model different scenarios and evaluate the trade-offs. The question is particularly relevant given the recovery of many marine mammal populations n many parts of the world following protection after exploitation or culling, combined with the ongoing challenge of managing fisheries in complex ecosystems. It also addresses a key aspect of ecosystem-based management: balancing multiple, sometimes conflicting, objectives within the same ecosystem.<\/p>\r\n\r\n

Model choices<\/h3>\r\nFor evaluation of impact of marine mammal rebuilding, Anchovy Bay has (only) an unspecified whale group and a seal group. In the basic model configuration (representing 1970), whales were slowly rebuilding while seals were in decline due to culling, which, however, stopped in 1972. \u00a0We can use the model to explore what would happen if we fit the model to time series up to the present (to let whales and seals rebuild), and then explore what impact these marine mammals have in the present situation. \u00a0You can download a fitted version of the Anchovy Bay model from this link<\/a>.\r\n\r\nTo evaluate the impact of marine mammals, load the model. You can simultaneously load the one scenario and time series file in the model, from the top menu as shown in Figure 1. Just click 1: anchovybay<\/em>, i.e. the time series, (which also loads the Scene 1<\/em> scenario).\r\n\r\n\"\"\r\n\r\nFigure 1. Loading Ecosim scenario and time series from the top menu.<\/strong>\r\n\r\nThen run the model (Ecosim > Output > Run Ecosim > Run)<\/em> and check the predation impacts of the marine mammals in the group plots (Ecosim > Output > Group plots > Predators ranked<\/em>). \u00a0Next, let's look forward, extend the duration of the simulation to 61 years at\u00a0Ecosim > Input > Ecosim parameters > Duration of simulation (years).\u00a0<\/em>Now extract results for the \"with marine mammals\" run at Ecosim > Output > Ecosim results<\/em>, for Fleets<\/em>, copy the columns with Fleet name<\/em>, Catch (end)<\/em>, and Value (end)<\/em> to a spreadsheet. From Group landed by<\/em>, copy the Group name<\/em>, Biomass (end)<\/em>, and Catch (end)<\/em> columns.\r\n\r\nNow, sorry, let's eradicate the marine mammals. At Ecosim > Input > Fishing mortality<\/em>, select the\u00a01: Whales<\/em> icon at the bottom part, then sketch a high fishing mortality, e.g., 0.5 year-1<\/sup> from 2012 onwards. Do the same for 2: Seals<\/em>, where you may have to sketch several times to get the Y-axis to go high enough. Run the model and extract the same results for this \"without marine mammals\" run as you did for the \"with marine mammals\" run. \u00a0Compare the two run, e.g, by calculating the ratio of \"without marine mammals\" to \"with marine mammals\".\r\n\r\nBut while <\/span>there's no discussion that Anchovy Bay is great, remember, models are not like religion \u2013 you can have more than one (and you shouldn't believe them). \u00a0Here, the <\/span>results from Anchovy\u00a0<\/span>Bay \u2013 which are quite drastic \u2013 begs the question, are they realistic? \u00a0To evaluate that, it would be pertinent to use a model that actually was designed to address the overarching policy question in this chapter.\r\n\r\nFor this, we can turn to a model published by Woodstock et al. \"Marine Mammal and Seabird Population Changes Have Contrasting but Limited Impacts on Fisheries Catches in the North Sea\"<\/em>.[footnote]Woodstock, M.S., Kiszka, J.J., Evans, P.G.H., Waggitt, J.J., Zhang, Y., 2025. Marine mammal and seabird population changes have contrasting but limited impacts on fisheries catches in the North Sea. Can. J. Fish. Aquat. Sci. 82, 1\u201314. https:\/\/doi.org\/10.1139\/cjfas-2025-0056<\/a>[\/footnote] \u00a0You can download the database from this GitHub link<\/a>.[footnote]Woodstock, Matthew (2025). Github repository for the paper: \"Marine Mammal and Seabird Population Changes Have Contrasting but Limited Impacts on Fisheries Catches in the North Sea\". figshare. Online resource. https:\/\/doi.org\/10.6084\/m9.figshare.30490466.v1<\/a>[\/footnote] \u00a0There are four EwE databases in the download, and for our testing we used the one named \"Mean Mammals_balanced.EwEmdb\", which uses average numbers of estimated marine mammals and birds. \u00a0We will refer to that model version in the following, but feel free to use any of the four model versions in the download.\r\n\r\nAs an option, you could base your analysis on the following scenarios.\r\n

1. Best estimate of ecosystem history<\/h4>\r\nThis is the Woodstock et al. (2025) scenario, which forces the marine bird and mammal biomass over time and otherwise fits the ecosystems groups to fisheries data to model the ecosystem history.\u00a0 Woodstock et al. compared competitive interactions between fisheries and marine mammals and birds over time by evaluating their relative mortality contributions over time.\r\n\r\nDownload the file from Github, open the Mean Mammals_balanced.ewemdb<\/em> model, and similar to in Figure 1, load the NS_mean<\/em> Ecosim scenario and the Mean_Time Series<\/em> time series. Run the model, Ecosim > Output > Run model > Run<\/em>. Extract the fleet and biomass results for the last year (\"end\") <\/em>from Ecosim > Output > Ecosim results<\/em> and copy-paste to a spreadsheet.\r\n

2. No rebuilding of marine mammals and birds: retrospective<\/h4>\r\nLoad the Ecosim scenario as above, but do not load the time series. If you check Ecosim > Output > Ecosim group plots<\/em>, the marine mammals and birds should now not be increasing as in the original run. \u00a0Similar to above, extract the results and copy to your spreadsheet.\r\n

3. No marine mammals or birds: retrospective<\/h4>\r\nThis more drastic (and hypothetical) simulation asking what would the situation be for the ecosystem and for fisheries if there were no marine mammals and birds in the (central and southern) North Sea?\r\n\r\nWe will evaluate this by removing marine mammals and birds from the system through an imaginary fishery. For this, introduce a new fleet, Fishery > Fleets > Define fleets<\/em>, and\u00a0Edit > Insert<\/em> to add a new fleet. Move the new fleet to the end #13 position, and call it, e.g., \"Culling\".\u00a0<\/em>Go to\u00a0Ecopath > Input > Fishery > Landings\u00a0<\/em>and enter a landing of 0.01 t km-2<\/sup> year-1<\/sup> for each of the marine mammals and birds (the first 9 groups).\r\n\r\nIf you add a fleet with this level of catches, you're bound to unbalance the model. To circumvent that, we can make a slight modification. \u00a0Remember the 2nd<\/sup> Ecopath Master Equation<\/a>,\r\n\r\n[latex]\\text{Production =\u2028 predation mortality\u2028 + fishing mortality \u2028+ biomass accumulation\u2028 + net migration \u2028+ other mortality}[\/latex]\r\n\r\nwhere we have just entered large fishing mortalities for the marine mammal and birds. We can balance the equation by adding a same-sized negative biomass accumulation for each group. In essence, we're saying that if there were large catches as we've entered, they would have resulted in the biomasses of the same groups being reduced with the corresponding amount. \u00a0When going to Ecosim, we can then set the effort for the Culling<\/em> fleet to 0 until we want the fleet to kick in.[footnote]We often use this trick to consider fleets that are introduced in a system after the Ecopath base year.[\/footnote]\r\n\r\nTo implement this, go to\u00a0Ecopath > Input > Other production<\/em> and set\u00a0Biomass accumulation (t km-2<\/sup> year-1<\/sup>)\u00a0<\/em> to -0.01 for each of the marine mammal and birds groups. Also go to Ecosim > Input > Fishing effort<\/em> and click Fleet 13, Culling<\/em> and Set to 0<\/em>. Run Ecosim again, and there should be no changes. \u00a0You can now click Set to value ...<\/em> and enter 1 to reset the effort to unity. Run again, and extract the fleet and biomass results for the last year from Ecosim > Output > Ecosim results<\/em> and cut-paste to a spreadsheet.\r\n

4-5. Rebuilding vs. culling: forward-looking<\/h4>\r\nAn alternative would be to take the current situation as the baseline and then project forward in time, thus focusing on the potential current trade-offs between fisheries and marine mammals and birds. \u00a0To set this up, locate and open the Mean_Time Series.csv<\/em> file in the GitHub timeseries<\/em> directory. \u00a0Make the following changes,\r\n
Scenario 4: Rebuilding projected<\/h5>\r\n