A century of change in a marine fish assemblage. Martin Genner

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1 A century of change in a marine fish assemblage Martin Genner School of Biological Sciences, University of Bristol Marine Biological Association, Citadel Hill, Plymouth

2 Demersal fishes of the English Channel

3 Annual hours fishing The demersal fishing industry of SW England Demersal fisheries operating for 1 years+ (Barrett et al. 24) Nearly all hook and line until widespread adoption trawling (18 th /19 th centuries). SW was instrumental in developing bottom trawling as an industry 1765 to 1767 Brixham company sent flatfish to London valued 2,8. By 179 fishers were heading offshore into deeper waters (Robinson 1998) English Channel hours fishing by UK registered vessels Year Steam Motor Sail Data: UK Government

4 Demersal fish landings (tonnes) Demersal fish landings (tonnes) Landings of demersal fish English Channel Plymouth Year Year Consistent fishing in the English Channel over the last century (with the exception of war years)

5 International Fisheries Exhibition in London, 1883 TH Huxley: I believe that it may be affirmed with confidence that in relation to our present modes of fishing, a number of the most important fisheries, such as the cod fishery, the herring fishery and the mackerel fishery, are inexhaustible. There are other sea fisheries of which this cannot be said. Theoretically, at any rate, an oyster bed can be dredged clean CM Mundahl: along the east coast a large quantity of immature cod are caught, and I firmly believe, notwithstanding what we have heard from Professor Huxley on the subject, that this wholesale capture of the young will sooner or later affect our supply of cod on the home grounds.

6 Catch per smack unit Catch (cwt) Evidence of declining catch rates changed perceptions Annual average catch of four Grimsby trawling smacks Plaice Haddock Others (prime & rough) Garstang, W. (19) The impoverishment of the sea. Journal of the Marine Biological Association of the UK Year Year

7 January-March ( C) July-September ( C) However the role of climate in driving change is uncertain Observed 5 year running mean Winter and summer sea surface temperatures of the English Channel have warmed consistently over the last 2 years. Grid square grid square (5 51 N, 4 5 W). Data: UK Hadley Centre.

8 MBA Long term data Standard Haul Trawl survey of the demersal fishes in the inshore waters off Plymouth (L4 area) Surveys began in 1911 (over 12 hauls) Ongoing, we aim for 2 hauls per month 85 demersal species sampled (+ pelagic species), over 775, individuals Abundance of species; Length, weight of individuals Possibly the longest running (but patchy) quantitative demersal fish time series in existence, enabling investigation of effects of climate change and fishing on the composition of a single marine fish assemblage.

9 MBA Long term data Standard Haul SS Oithona RV Salpa RV Sabella RV Sula RV Sarsia 1979 RV Squilla RV Plymouth Quest 24-

10 MBA Long term data Standard Haul 4 minute tow, fine mesh cod end cover All individuals identified, measured and weighed

11 MBA Long term data Standard Haul 6ºN 55ºN 4º3 W 4º15 W 4º W PLYMOUTH 5ºN 5º 2 N 1ºW 5ºW º 5 o 1 N

12 Analyses based on a core 3 demersal species Predictions Effects of fishing (size-selective harvesting) Declines in length/mass of species? Decline in abundance of species? Effects greater in most vulnerable species (large, slow maturing, high trophic level)? Effects of climate Warming periods favour southern warm water species? Most pronounced in small fast-growing species with intrinsically high population growth rates?

13 Mean annual abundance (individuals per hour) poor cod (Trisopterus minutus) red mullet (Mullus surmuletus) Abundance of smaller fish significantly related to sea temperatures the previous winter (P =.9), but not abundance of larger (predatory) species. Atlantic cod (Gadus morhua) spotted ray (Raja montagui)

14 Catch per hour (individuals) Mass (g) Estimated mass change (slope, g yr -1 ) Trait-dependent abundance and size changes 3 John dory P <.5 12 Thornback ray Maximum mass (log 1 g) Catch per hour (individuals) Mass (g) Abundance change ( ) Change in mass (g/yr) P < Year Maximum mass (log 1 g) Year

15 Catch per hour (individuals) January-March ( C) Climate association (correlation CPUE x winter SST focal year) Example common dragonet (Callionymus lyra).6 P < , 1, Maximum body mass (g) Stronger responses to climate in smaller fishes Latitudinal limits not as good for prediction of thermal associations (P =.8). Most species responding are not new climate change affects our core species Year

16 Mass change (g yr -1 ) Climate association (correlation CPUE x winter SST focal year) Abundance trend (correlation CPUE x year).8 Dichotomous responses to fishing and climate change Larger species : abundance declines , 1, Maximum body mass (g) Small species : abundance correlates strongly with temperature changes , 1, Maximum body mass (g) 1 Larger species : declines in body size , 1, Maximum body mass (g)

17 PC1 (mean and range) PC1 (Community Structure) PC1 (mean and range) a) Small species (29% of variation) b) Large species (17% of variation) Year Small species (< 9g max mass) PC1 not correlated with large species PC1 (P =.126). Small species PC1 significantly associated with winter SST (multiple regression; model P <.1).

18 Predicted effects of fishing (size-selective harvesting) Declines in length/mass of species - YES Decline in abundance of species - YES Effects greater in most vulnerable species (large, slow maturing, high trophic level - YES Predicted effects of climate Warming periods favour southern warm water species PARTLY (large species affected by fisheries?) Effects most pronounced in small fast-growing species YES A trait based approach enables us to disentangle the effects of fishing and climate of demersal fishes

19 Genner et al. (21) Body size-dependent responses of a marine fish assemblage to climate change and fishing over a century-long scale. Global Change Biology doi: /j x Acknowledgements: Main collaborators: David Sims, Matthew McHugh, Stephen Hawkins Ships captains and crews, past and present. Funding: