Tutorial: Adding multi-stanza groups (edit to only add multi-stanza)

The purpose of this exercise is to demonstrate how to add multi-stanza groups to an EwE model.

The model as defined for previous tutorials in this text book had these functional groups:

Whales, seals, cod, whiting, mackerel, anchovy, shrimp, benthos, zooplankton, phytoplankton, detritus.

Start by opening EwE6, select File > New model. Browse to your preferred file location, and enter a name for the model. For instance, “Anchovy Bay”.  Now navigate to Ecopath > Input > Basic input. The model will have one group, Detritus. All models must have a detritus group, so we have entered it for you. Why? We need to be sure there is a group where we can send excreted and egested material as well as dead organism. By default they go to the detritus group.

On Ecopath > Input > Basic input select Define groups (also available from the menu on top: Ecopath > Define groups). Click Edit > Insert on the right side of the form that pops up. Continue clicking till you have 12 groups; then enter the group names, i.e., Whales in first row, Seals in second, etc. [Hint: you can cut and paste the names from Excel, using Ctrl-C and Ctrl-V]. We will make mackerel an age-structured (‘multi-stanza’) group, therefore enter “Mackerel” under the “Multi-stanza group name” for Mackerel juv., then use the down arrow in the row for Mackerel ad. and select “Mackerel” from the drop-down menu (this is to make sure that the stanza name is spelled the same way). Also set the Stanza age to 0 months for the juveniles, and 3 months for the older stanza, (here called “adults”). When you have entered all, click the Producer check mark in the phytoplankton row. On the right panel, you may also want to click the Colors, Alternate all, to change the group colors to your liking. Click OK.

We also need to define our fishing fleets. Click Fishery on the Navigator to the left. Then click Definition of fleets, and then Edit fleets above the spreadsheet (or click Ecopath on the top menu, and then Edit fleets). We need four fleets: sealers, trawlers, seiners, foragers, and shrimpers. We can enter catches while we are here; unit should be t km² year^-1. The sealers caught 15 seals in 1970 with an average weight of 30 kg. The fisheries catches were 45 t of cod and 20 to of whiting for the trawlers, 40 t of mackerel ad. for the seiners, and 140 t of anchovy for the foragers, and 3 t of shrimp for the shrimpers. Calculate catches using the appropriate unit, and enter in EwE.

The off-vessel landing prices are seals 6 $/kg; cod: 10 $/kg; whiting $6/kg; mackerel ad: 4 $/kg; anchovy is 3 $/kg for foragers. Shrimps are 20 $/kg. Prices are current prices (hence “are” instead of “were”) as we later will be using these for forward projections.

If you lack catch or price information for your own models later, then check www.seaaroundus.org.

We now should enter the basic input parameters. Fortunately, the biologists have been busy, and we have some survey estimates from 1970 of biomasses in the bay. The biomasses must be entered with the unit: t/km2.

Whales: 10 individuals with an average weight of 800 kg. Seals: 203 individuals with an average weight of 30 kg. Cod: 300 t. Whiting 180 t. Mackerel ad.: 120 t. Anchovy: 640 t. Shrimp: .16 t/km2. Zooplankton: 14.8 t/km2.

Next are production/biomass ratios, which with certain assumptions (that we don’t worry about now) correspond to the total mortality, Z. The unit is year-1, and we can often get Z from assessments. Alternatively, we have Z = F + M, so if we have the catch and the biomass, we can estimate F = C/B and add the total natural mortality to get Z.

We do this for the fish where we can get an estimate of M and Q/B from FishBase.org. Search for the species, (Gadus morhua, Merlangius merlangus, Scomber scombrus, Engraulis encrasicolus), and extract the values. Estimate Z = F + M.

It is also an option for exploited species to use an equation for estimation of Z that was developed by Beverton and Holt (1957). It is implemented in the life-history table in FishBase. It relies on estimates of length at first capture (Lc), average length in the catch (Lmean), and asymptotic length (Linf) to estimate Z. Try it for the four species here. Here are the lengths from the fishery in Anchovy Bay:

 Compare the Z estimates from the two methods (and consider).

There is a close relationship between size and P/B; the bigger animals are, the lower the P/B. Here we have: Whales: P/B = .05 year-1; seals: get F from catch, and M is .09 year-1; shrimp P/B = 3 year-1; benthos P/B = 3 year-1; zooplankton: it is mainly small Acartia-sized plankton, with P/B = 35 year-1.

We can get P/B for many invertebrates from Tom Brey’s work (but don’t need to for this tutorial). Check out: http://www.thomas-brey.de/science/virtualhandbook/. There is a neat collection of empirical relationships and conversion factors.

Consumption/biomass ratios for the non-fish groups: for whales use 9 year^-1, and for seals 15 year^-1. For the invertebrates enter a P/Q ratio of .25 instead of entering a Q/B. Finally, there is phytoplankton. We can often get primary production estimates from SeaWiFS satellite data. Here we have PP = 240 gC m² year^-1. The conversion factor from gC to gWW is 9, so an easy way to parameterize this is to enter a biomass of 18 t km^-2 for phytoplankton and a P/B of 120 year^-1. EwE only uses the product of these, so it doesn’t really matter how they are distributed, but the P/B indicates a turnover of less than once per day, which is reasonable. [You could also use B=9 t km^-2 for phytoplankton and a P/B of 240 year^-1, but such a high P/B can make Ecospace run unstable[

Next parameter is Ecotrophic Efficiency (EE), this is the part of the production that is used in the system (or rather for which the model explains the fate of the production). In this model, we are missing a biomass estimate for benthos. We do not explain much of the mortality for this group, so we guess an EE = 0.6. For the other groups, we let Ecopath estimate the EEs, but bear in mind the definition of EE when you evaluate the estimated parameters.

In the Ecopath baseline year, the whale population had started to recover after whaling, but the seal population was still declining, so the Ecopath baseline model is not in steady state. We specify this on the Input data; Other production form by entering a biomass accumulation rate of 0.2 year-1 for whales, and -0.05 year-1 for seals.

Next we parameterize the age-structured model for mackerel. For this click the baby pram / Edit multi-stanza Ecopath > input > Basic input form. Set the growth curvature parameter to 0.3 year^-1 for mackerel (= the average k value from FishBase); total mortality for Mackerel juv. should be 4 year^-1, and “leading” should be checked for Mackerel ad. for both biomass and consumption/biomass. Click Calculate, and the model should parameterize the age structured model.

Now it’s time for diets:

With this we have the information that is needed to mass-balance the model. Select Parameterization, Basic estimates, and check out the outcome. Save the model.

Try changing some of the input and see what happens. Don’t save afterwards.

Go to Ecosim > Output > Run Ecosim  Click Run, and see what happens.