The state of the fish community in ospar region 2, The Greater North Sea

Sub-Regional Assessments (Table 2)

Northern North Sea

In the Northern North Sea, trends in the 15 univariate community metrics suggested little change in the composition, structure and function of the demersal fish community over the last decade; a decrease in the abundance/biomass/productivity aspect was indicated, which was interpreted as beneficial. Over the longer period, both species richness and species evenness are currently higher than when data were first available for analysis, but the community is has become more dominated by smaller sized fish.

Southern North Sea

In the Southern North Sea, demersal fish species richness and species evenness have increased over the last decade, but the community has at the same time become more dominated by fish belonging to species with opportunistic life-history trait characteristics. Size composition and abundance/biomass/productivity show no systematic trend over the last ten years. However, compared with the mid to late 1980s, community size composition has shifted towards smaller sized fish and species richness has increased. Metric values for the other three aspects of demersal fish community’s composition, structure and function are little different now to those prevalent when data first started to be collected.

Eastern Channel

A possible methodological problem makes assessment of the 15 univariate trends difficult in the Eastern Channel sub-region. However, taking the data at face value suggests that over the last decade the size composition of the fish community has improved, particularly in the last few years. But little in the way of any systematic trend was apparent in any of the four other aspects of the composition, structure and function of the demersal fish community. Interpretation of longer-term trends was most problematic. Here there was some suggestion that species evenness and species richness may have declined, but this could easily be an artefact caused by the possible methodological problem. The data suggest that other aspects of the community’s composition, structure and function are little different now than in the late 1980s to early 1990s.

Western Channel and Celtic Sea

Over the recent decade, species richness, species evenness, and life-history trait composition of the demersal fish community in the Western Channel and Celtic Sea sub-region have all improved so that the current state is now better than during the mid to late 1980s, when data were first available. However, the size composition and biomass/abundance/productivity metrics all suggest long-term deterioration in these two aspects of the composition, structure and function of the community, although the decline in size composition may have ceased over the last decade.

Irish Sea

The state of the demersal fish community in the Irish Sea has improved over the last decade, with metrics related to biomass/abundance/productivity, species richness, species evenness, and size composition aspects of the community all indicating an improvement, while life-history trait metrics showed little change. As a result, the community is now in better state than during the 1990s when surveys started. Improvement was indicated in all aspects of the community’s composition, structure and function except for biomass/abundance/productivity, for which the situation now is similar to that when data were first available.

Minches and Western Scotland

Over the recent decade, trends in the fifteen metrics suggest improvements in all five aspects of the composition structure and functioning of the demersal fish community in the Minches and Western Scotland sub-region. As a consequence of which, the abundance/biomass/productivity, size composition, and species richness aspects are now on a par with the situation observed when data were first available, while the species evenness and life-history composition aspects would appear to be better.

Scottish Continental Shelf (southwest)

Over the recent decade, trends in the fifteen indicators suggest improvements in all five aspects of the composition structure and functioning of the demersal fish community in the southwestern sector of the Scottish Continental Shelf sub-region. As a consequence of which, the abundance/biomass/productivity and size composition aspects are now on a par with the situation observed when data were first available, while the species evenness and life-history composition aspects would appear to be better. However, despite recent improvements, demersal fish species richness is still currently lower than at the start of the time series.

Scottish Continental Shelf (northeast)

Trends in the fifteen univariate community metrics in the northwestern sector of the Scottish Continental Shelf sub-region portray a picture of variability, but very little systematic trend. Over the last decade no obvious trend was apparent in any of the five aspects of community composition structure and functioning. Over the longer term, only species richness showed any real difference, being slightly higher now than at the start of the time series.

Rockall trough and Bank

Metric trends over the last decade impart a mixed message. The species diversity and size composition aspects of the Rockall Bank demersal fish community have improved, whilst abundance/biomass/productivity appears little changed. However, the life-history trait composition metrics suggest a decrease in “climax community” species and an increase in the abundance of “opportunistic” species. A similar story emerges with regard to longer-term change in the composition, structure and function of the Rockall Bank demersal fish community, except that recent changes in size composition have simply achieved parity with the earlier data. Given the scarcity of data, particularly the gap between 1986 and the start of the systematic surveys in 2001, confidence in these assessments is low.

Overall Regional Assessment

Over the last ten years, all aspects of the composition, structure and function of the demersal fish community in UK waters, with the exception of life-history trait composition, have improved. Life-history trait composition has shown little or no change. As a result of these changes, species diversity (richness and evenness) of the UK’s demersal fish communities is now generally better than during the early 1980s and 1990s, abundance, biomass, productivity and life-history trait composition are about on a par, but size composition is still disturbed towards smaller fish. Improvement in this latter aspect is still therefore necessary, requiring that the current improving trend be maintained.

Metric	Abbreviation	Metric calculation	Terminology
Biomass	B		Where S is the total number of species, l is the length class, Ns,l is the total number of individuals in each length class of each species. The constants cs and bs are the constant and exponent values respectively in the species-specific weight at length relationship.
Abundance	N		Where S is the total number of species, l is the length class, and ns,l is the number of fish in each species and length class.
Daily growth production	P		Where S is the total number of species, l is the length class, Ns,l is the total number of individuals in each length class of each species. The constants cs and bs are the species-specific weight at length relationship constant and exponent values respectively. The constants ks and l∞,s are the species-specific von Bertalanffy growth function growth and ultimate body length values respectively. ks is divided by 365 to convert and annual parameter to a daily parameter.
Daily production to biomass ratio	P/B		Where P is the total daily growth production and B total biomass of the fish community (see above).
Large fish indicator	LFI		For term explanations, see “Biomass” above. Note that in the numerator, the summation is carried out across lengths >40cm only.
Mean weight of fish	W		Where B is the total biomass and N the total number of fish in the sample (see above).
Species count	S	S	Where S is the count of the number of species in the sample.
Margalef’s species richness	SMarg		Where S is the total number of species and N the total number of individuals in the sample (see above)
Pielou’s evenness	J		Where Ns is the number of individuals belonging to species s, N is the total number of individuals of all species in the sample, and where S is the total number of species recorded in the sample (see above).
Hill’s N1 diversity	N1		Where Ns is the number of individuals belonging to species s, N is the total number of individuals of all species in the sample, and where S is the total number of species recorded in the sample (see above).
Hill’s N2 dominance	N2		Where Ns is the number of individuals belonging to species s, N is the total number of individuals of all species in the sample, and where S is the total number of species recorded in the sample (see above).
Mean ultimate body length	L∞		Where l∞,s is the von Bertalanffy ultimate body length of each species s. S is the total number of species recorded in the sample and Ns is the total number of individuals of each species caught. N is the total number of individuals recorded in the sample.
Mean growth coefficient	K		Where k,s is the von Bertalanffy growth parameter for each species s. S is the total number of species recorded in the sample and Ns is the total number of individuals of each species caught. N is the total number of individuals recorded in the sample.
Mean length at maturity	Lmat		Where lmat,s is the length at maturity of each species s. S is the total number of species recorded in the sample and Ns is the total number of individuals of each species caught. N is the total number of individuals recorded in the sample.
Mean age at maturity	Amat		Where amat,s is the age at maturity of each species s. S is the total number of species recorded in the sample and Ns is the total number of individuals of each species caught. N is the total number of individuals recorded in the sample.

Table 1. Descriptions, abbreviations and derivations of the fifteen univariate community metrics applied to the groundfish survey data.

Aspect of Fish Community Composition, Structure and Function	Abundance, Biomass and Productivity	Size Composition	Species Richness	Species Evenness	Life-history Trait Composition
Assessment of Trend Since 1999
Northern North Sea
Southern North Sea
Eastern Channel
Western Channel and Celtic Sea
Irish Sea
Minches and Western Scotland
Scottish Continental Shelf (southwest)
Scottish Continental Shelf (northeast)
Rockall trough and Bank
UK EEZ ASSESSMENT	2.362	2.447	2.609	2.609	1.843
Comparison of State of Fish Community in Last Five-year Period with First Eight-year Period in Time Series
Northern North Sea	1983-1990	1983-1990	1983-1990	1983-1990	1983-1990
Southern North Sea	1983-1990	1983-1990	1983-1990	1983-1990	1983-1990
Eastern Channel	1988-1995	1988-1995	1988-1995	1988-1995	1988-1995
Western Channel and Celtic Sea	1984-1991	1984-1991	1984-1991	1984-1991	1984-1991
Irish Sea	1992-1998	1992-1998	1992-1998	1992-1998	1992-1998
Minches and Western Scotland	1986-1993	1986-1993	1986-1993	1986-1993	1986-1993
Scottish Continental Shelf (southwest)	1986-1993	1986-1993	1986-1993	1986-1993	1986-1993
Scottish Continental Shelf (northeast)	1983-1990	1983-1990	1983-1990	1983-1990	1983-1990
Rockall trough and Bank	1986	1986	1986	1986	1986
UK EEZ ASSESSMENT	1.881	1.617	2.719	2.749	2.153

Table 2. Assessment of changes in the composition, structure and function of the demersal fish community in each sub-region of the UK’s EEZ and overall assessment for the entire EEZ. Green cells indicate beneficial changes. Red cells indicate detrimental changes. Where no discernable trend is apparent, cells are coded orange. Sub-regional “reference periods” are indicated in the lower half of the table. Values in the overall regional assessment rows indicate weighted average scores. Two assessments for the Scottish Continental Shelf are shown; northeast refers to the IBTS based assessment and southwest refers to the assessment based on the Scottish West Coast Groundfish Survey.

Figure 1. Chart showing ICES rectangles assignments to Charting Progress II sub-regions. (Northern North Sea – red; Southern North Sea – yellow; Eastern Channel – blue; Western Channel and Celtic Sea – green; Irish Sea – orange; Minches and Western Scotland – purple; Scottish Continental Shelf – pink; Rockall Trough and Bank – grey; Faroe-Shetland Channel – black). Red line indicates sub-division of the Scottish Continental Shelf sub-region for analytical purposes.