6 Biomasses and units
Though biomasses to some extent can relate to “real estate” the issue is not “location, location, location”, but “units, units, units”. When you work with ecosystem models, you’ll have to obtain information for a multitude of sources and they will be using different units that need to be converted – and that often leads to conversion errors.
Units are important, and one really important thing that Ecopath has contributed has been to force (or maybe entice is a nicer word) modellers to standardize biomasses to a per unit area basis. So, for biomasses, the standard unit is ton per square kilometre in Ecopath. That makes it straightforward to compare abundance between ecosystems, where using total tons in the overall ecosystem does not just because of differences in physical ecosystem size.
How do you then get biomasses for your model? Fortunately, biomasses are standard output from surveys and assessments, and we will refer to that literature without being much more specific about how to obtain biomasses. As a rough classification, note that there are “direct” estimation methods such as trawl swept area, acoustic target expansion, visual census, plankton sampling, and “indirect” methods where biomass are output from assessments that use multiple sources of information to estimate biomasses. The biomasses add constraints to your model, and constraints make the model outputs appear to be less uncertain.
If at all possible, get biomasses from local sources (i.e. for your ecosystem), and be aware that biomasses “don’t travel well”. It helps that we are using per unit area biomasses, but conditions really vary from system to system due notably to differences in productivity and fishing pressure over time.
The million dollar question
How do you convert from t km-2 to g m-2? The answer is: they are equal.
So, when you evaluate model parameters, think t km-2 for the big things, and g m-2 for the small.
For instance, this bay is around 100 km2 and there are some 100 seals each with a weight of 50 kg. That’s 50·100 kg = 5 t in 100 km2 = 0.05 t km-2. Or, if we assume there’s 3 shrimps per m2, each weighing 2 g; then the biomass is 6 g m-2 = 6 t km-2 as we know now.
That simple conversion between t km-2 and g m-2 really makes it simple and elegant to relate to biomass estimates for all kind of critters in an ecosystem. But watch out because biomasses for smaller critters (zooplankters, benthic invertebrates, insect larvae) are often reported in “dry weight” units without information on the drying protocol, and must be converted to the wet weight units typically used for larger critters like fish. The wet/dry weight ratio can vary from as low as 5 to over 10.
There is an ongoing controversy about whether one can or should use output from one model (assessment) as input for another model (ecosystem) where the sentiment from assessment scientists may be a No! and that we should instead use the same input (surveys) as used for assessments.
This argument ignores the fact that all biomass estimates are in fact based on models, i.e. on various aggregation, transformation, and calibration scaling operations applied to raw data. So working with the raw data as inputs would mean not just repeating one assessment but all estimations done for the ecosystem, including estimates of primary productivity and other supposedly “direct” measurements. Primary productivity (phytoplankton biomass) estimates for marine systems in particular are typically based on complex models evaluating satellite information, with calculations so specialized that it would make no sense to try to repeat them. Acoustic abundance estimates are similarly complex expansions of raw target data. Even simple swept area or volume conversions from nets are fraught with uncertainties about conversion factors. The same uncertainties more obviously hold true for assessment models, for which we have to at least initially hope that the panels reviewing the models have weeded out really bad estimates.
But thankfully, the Ecopath biomass estimates are not carved in stone; just as we typically do with single species assessment models, we can vary the Ecopath input values and examine how that variation affects dynamics, policy responses, and likelihood measures of goodness of fit to available time series data on relative abundance trends and outputs (like catches).
There are at least two good reasons to use most recent biomass estimates from single species models as the Ecopath input biomass estimates. First, those Ecopath estimates are used to initialize time simulations with Ecosim, providing a capability to predict forward from the most recent assessment estimates using a model that is initially consistent with the assessment model but explicitly represents trophic interactions evident in the Ecopath inputs when looking forward over time. Second, Ecopath can provide a credibility check on the single-species model estimate, in particular whether the estimate is high enough to support estimated predation rates on it (Ecotrophic efficiency less than 1.0), and whether prey abundance is high enough to support its estimated food consumption. But be warned: it is not so easy to defend the use of estimated biomasses for early years from single species assessments to set Ecopath base biomasses for such early years, because of uncertainties in the single species results about cumulative net depletion of stock size over time due to historical removals and other factors.
