Identification of inshore nursery areas for commercially important fish species around Shetland

Coastal environments provide many ecologically significant services, delivered through high productivity and diversity. As fish habitats, nearshore environments are considered most valuable to juvenile fish where shallow, sheltered areas function as nursery habitats. Fish nurseries which demonstrate considerable value to adult fish stocks through improved growth and survival are considered a component of essential fish habitats (EFH). Given that the abundance of adult fish populations can be largely determined by the strength of early juvenile age-classes, identifying and assessing the significance of EFHs is vital to the management of commercial fisheries and to understanding the wider marine ecosystem.

The population age structure of fish and potential nursery habitats in nearshore waters has not previously been studied in detail in Shetland. The annual Shetland Inshore Fish Survey (SIFS) is a unique source of inshore data which provides an opportunity to estimate the age structure of nearshore fish populations, investigate their population dynamics, and to identify potential nursery grounds based on consistent use of specific areas by juveniles through time.

Species for analyses were selected based on commercial relevance and those sufficiently resolved in the available data. Specifically, these were Atlantic cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus) and plaice (Pleuronectes platessa). Age-length keys (ALKs) for these species were produced from data available from the North Sea component of the International Bottom Trawl Survey (NS-IBTS). ALKs from NS-IBTS were selected for the area corresponding to Shetland (Area 1) in the third quarter (Q3) of 2017-2024 and applied to the observed length data from SIFS. From this, the age-structure of cod, haddock, whiting, and plaice inshore populations around Shetland were modelled, and the relative abundance and spatial distribution of discreet age-classes were determined.

Juvenile subpopulations for all four species were identified in coastal areas. High concentrations of age-0 fish were shown to consistently inhabit shallow and sheltered voe environments which were often clustered in specific areas. Three clusters were identified as being particularly important nursery habitats for multiple species, these were: the Weisdale Voe and Sandsound Voe to the south of the west mainland; Lunna and Dales Lees off the east mainland; and Cole Deep and Skeetlie (Aith Voe) to the north of the west mainland. Some other nearshore areas were also identified as having high persistence rates of age-0 fish for one or more species (e.g. age-0 cod were present at Ollaberry in 100% of survey years), and juvenile plaice were more widespread but with lower catch rates than the other species considered here.

For gadoid species (i.e. haddock, cod, and whiting), high age-0 catches were recorded in coastal areas where adults from the same species were absent. This evidence of spatial separation of juvenile and adult populations further supports the classification of the identified areas as nursery habitats. The environmental characteristics of the identified nursery habitats include shallow depths (<50m), shelter from wave action and tides, and various (often sandy) sediments with some evidence of macroalgal (i.e. seaweed) cover. The available data indicates offshore movement of growing fish from these nursery habitats towards deeper fishing grounds which highlights the potential significance of these areas to nearby fish stocks and to local fisheries.

This study concludes that it is more important than ever to maintain and continue valuable time series such the Shetland Inshore Fish Survey, especially given the unprecedented scale of ongoing and proposed industrial developments in the Shetland region. Empirical data such as those presented in this report are vital for understanding complex nearshore environments and identifying essential fish habitats. It is recommended that annual monitoring is continued and that further research is carried out to fully investigate the extent and significance of the nursery areas identified here and to detect any changes in fish populations and habitat use in the future.