Using a residency index to estimate the economic value of saltmarsh provisioning services for commercially important fish species

Every year, 100 hectares of saltmarsh in the United Kingdom are lost due to sea level rise. The remaining areas are threatened by land conversion, agricultural activities, and climate change. There are important economic consequences to saltmarsh loss, as saltmarsh provides valuable ecosystem services including flood protection, carbon sequestration, and nursery habitat for commercially fished species. Quantifying the economic value of these ecosystem services can help target policies for saltmarsh restoration, or ‘managed realignment’, of new saltmarsh areas. In this study, we quantify the economic value of saltmarsh as a habitat for commercially fished species by developing a residency index. The residency index weights the relative importance of saltmarsh along a species’ lifecycle by explicitly incorporating the target species’ life histories and the estimated proportion of time it spends in saltmarsh at juvenile and adult life stages. Using this index, we estimate the value of saltmarsh to UK commercial fisheries landings. We find that UK saltmarsh contributes annually between 16.7% and 18.2% of total UK commercial landings for European seabass (Dicentrarchus labrax), European plaice (Pleuronectes platessa), and Common sole (Solea solea). Our findings highlight the importance of saltmarsh protection and restoration. Furthermore, our approach provides a general framework that integrates population ecology methods and economic analyses to assess the value of saltmarsh and other coastal habitats for fisheries worldwide.


INTRODUCTION 20
Globally, coastal-habitat extent is in severe decline (Waycott et al. 2009; Barbier et al. 2011;21 Balke et al. 2015). This decline is caused by a variety of human-related activities including 22 climate change, runoff, and coastal development (Waycott et al. 2009;Balke et al. 2015). 23 Rates of decline vary between different coastal habitats. For example, more mangrove forests 24 (35%) have been lost or degraded worldwide, than coral reefs (30%) or seagrass communities 25

Calculating saltmarsh residency indices 101
Following Scott (1999), we assumed that saltmarsh residency is a proxy for saltmarsh 102 dependency. This allowed us to formulate an index quantifying fish dependency on saltmarsh 103 based on earlier work by Scott (1999): Because not all five exact species are currently available in the COMADRE database, we used 127 matrix population models for closely related species and with similar life history traits (see 128 The demography of each species in COMADRE is archived into associated sub-matrices.  Table 1), 146 we first collapsed each matrix into a 2x2 model containing a juvenile and an adult stage 147 following the method described by Salguero-Gómez & Plotkin (2010). All initial stages prior to 148 the first reproductive stage were considered juveniles, and all reproductive stages were considered adults. Next, we used methods by Caswell (2001) and Cochran & Ellner (1992) to 150 obtain the age-specific survivorship curve (lx). We used the survivorship curve to calculate 151 age-specific mortality rates (1-lx) and used the mean of the linear mortality models at each 152 stage to represent stage-specific mortality. We calculated natural mortality ( ) rates by 153 averaging these age-specific linear mortality models across the entire lifespan. We also 154 quantified the juvenile residence time ( ) and adult residence time ( ) by calculating the 155 fundamental matrix, also following methods by Caswell (2001). 156

Estimating proportion of time spent in saltmarsh 157
There are few quantifiable, habitat-specific demographic studies for fish (Vasconcelos et al.  one of which is saltmarsh, we assume that it spends 25% of its time in saltmarsh. This 191 assumption simplifies species' habitat use, and therefore introduces the possibility of under-192 or over-estimating the importance of saltmarsh compared to other habitats. We later explore 193 the implications of this assumption through a sensitivity analysis. 194 We conducted a systematic meta-analysis of habitat use for each study species for all years 195 available in Web of Science and SCOPUS. For full search strings, see Appendix S2. The 196 criteria for inclusion of a peer-reviewed publication was that it must (i) be written in English,197 (ii) focus on at least one of our five study species, and (iii) provide habitat use information.

Commercial fisheries landings 215
To obtain information regarding yearly landings for each species in UK waters, we used ICES 216 time series catch data (ICES 2018). These data identify the species, the ICES area in which 217 the fish were caught, and the live weight in tonnes. We considered catch data from nine ICES

Applying the Saltmarsh Residency Index 234
To apply the saltmarsh residency index to landings values, we multiplied the SRI value for 235 each of our five target fish species by the commercial fisheries value for that species. We then 236 added these together to quantify the total commercial value for saltmarsh (Jackson et al.,

Sensitivity Analysis 244
We conducted a sensitivity analysis to test the responsiveness of the final commercial value 245 of saltmarsh to different estimates of the proportion of time adults and juveniles spend in 246 saltmarsh. We focused on this time proportion as it is a key element of the assumption 247 underpinning our analysis: that residency is a proxy for dependency (Scott 1999). To conduct 248 this analysis, we calculated a species-specific sensitivity index for incremental changes in the Commission 2018). The contribution of UK saltmarsh to this total was between £23.9 million 267 and £25.66 million, or between 45.6% and 49% of the 2015 total. These five species are 268 estimated to spend between 6% and 22% of their juvenile life stage in saltmarsh, where they 269 are protected from predators and have access to safe, plentiful feeding grounds (Green et al. The provisioning services provided by UK saltmarsh to UK commercial fisheries were highest 272 for P. platessa, with its estimated value attributed to saltmarsh ranging from £1.9 million to 273 £2.4 million (26.5% to 33.5% of its total commercial landings value) (Figure 2). This large 274 contribution can be attributed to two factors: 1) this species has the highest average SRI value 275 of all 5 species (0.3, see Table 2, & Figure 2) and 2) an average yearly catch rate at least six 276 times higher than any other species. The species with the lowest economic contribution was 277 D. labrax (besides C. labrosus and C. ramada, which had no recorded UK landings). This is 278 likely due to low catch levels in 2015 caused by catch limits set by the EU, which meant that 279 its potential saltmarsh economic contribution was low (European Commission n.d.). 280 The calculated saltmarsh residency index (SRI) values (Table 2)  In this study, we exemplify a species-specific approach to estimate the value of saltmarsh for 300 commercial fisheries. Our approach demonstrates how demographic and economic modelling 301 can be effectively combined to more accurately estimate saltmarsh ecosystem services value. 302 We found that the commercial value of UK saltmarsh to the target species, European seabass 303 government resources for managed realignment projects, it is essential that both the 386 ecological benefits, as well as the socio-economic benefits of these projects are realistically 387 and accurately estimated. Using two different approaches, we demonstrate a region-specific, 388 species-specific method for more accurately estimating the value of saltmarsh for fisheries. 389 Regional-specific guidance for estimating the economic benefits of coastal habitats, as 390 presented in our analysis, can help guide policy makers to make decisions that maximise 391 social and ecological outcomes.