DYNAMIC THINNING LINES, A UNIVERSAL CONCEPT ON PLAICE NURSERY GROUNDS . Nash, R.D.M. 1 , Geffen, A.J. 1,2 , Witte, J. IJ. 3 , and van der Veer, H.W. 3 1 Institute of Marine Research, PO Box 1870 Nordnes, 5817 Bergen, Norway 2 Department of Biology, University of Bergen, PO Box 7800, 5020 Bergen, Norway 3 Royal Netherlands Institute for Sea Research, NIOZ, PO Box 59, 1790 AB Den Burg Texel, The Netherlands
1.Nursery ground context 2.What are self-thinning/dynamic thinning and boundary lines 3.The general dynamics of the early life history of plaice 4.Why plaice nurseries are ideal for field studies 5.Examples of self-thinning/dynamic thinning lines in plaice nurseries 6.The relevance of this concept for understanding nursery ground dynamics
The dynamics: Numbers of individuals on a nursery ground seasonally increase, individual weights increase, total biomass increases, at some point density begins to fall and then total biomass decreases until the influx of the new year class and the cycle starts again. Specifically considering the cases where the population reaches the Numbers carrying capacity Individual weight Total biomass Time (April to December)
So what is self-thinning? From plant ecology: Crowded, even-aged monocultures approach and then track along a line. = w c . d - 3 / 2 Where: w = mean weight, d = population density and c = constant Total weight of the population continues to increase as individuals grow, self-thinning occurring until resource limitation or structural or physiological constraints cause a cessation in increase. Total weight then remains constant i.e. ‘ carrying capacity ’ is reached and the slope becomes –1 on a log-log plot.
In diagrammatic form (plants): Slope = -1 (Constant biomass) Resource limitation, structural or physiological constraint Log Slope = -3/2 mean Self-thinning boundary weight Initial growth Log density
The self-thinning rule: as applied to animal populations Expected that a constant biomass/carrying capacity might be applicable i.e. slope = -1. However, relating the slope to metabolic rates i.e. raised to 0.75 or a slope of –1.33 is probably more applicable for mobile animals. This gives: = - log w c 4 / 3 log d Proviso: Assumes that the food/resource (F) input to the population remains constant throughout the growth of the cohort. However, the possibility that F remains constant for animal populations is much less likely.
Self-thinning in animal populations: Theoretical considerations Amount of food increases with time. Steepens the s-t line (<-4/3 e.g. –3/2, -2 etc) dF +ve dt Log Mean weight Log density Adapted from Begon et al. 1986
Self-thinning in animal populations:Theoretical considerations Consumption outstrips food growth Less steep dF –ve (>-4/3 e.g. –1) dt Log Mean weight Log density Also: dF/dt can change due to the behaviour of the population Adapted from Begon et al. 1986 e.g. territoriality or migration
Boundary lines Species boundary lines: ‘a line beyond which combinations of density and mean weight are not possible’ (see Weller 1990, Begon et al. 2006) Essentially the carrying capacity of the environment. Food-limited cohorts The carrying capacity is determined by the balance between growth and mortality
Self-thinning in food-regulated populations [energetic equivalence theory] Carrying capacity is where βg=m (intercept). Lower food production affects m Growth (g) either βg or m. Mortality (m) F Equilibrium energy flow (F) is lower in habitats with lower βg food production. If food production keeps changing then the slope will Log (N) change. In seasonal environments the slope is not constant Log (w) Progress along the thinning lines are equal. Bohlin et al. 1994
The case of energy flow (F) varying with body size Curvilinear thinning lines Log (F/k) Diet changing with body size Log (N) Food for intermediate body sizes having a higher rate of production Log (w) Bohlin et al. 1994
Why choose juvenile flatfish and plaice in particular? Kristineberg Marine Research Station Port Erin Marine Laboratory Dunstaffnage Marine Laboratory NIOZ
Early life history trajectory for North Sea plaice ( Pleuronectes platessa ) Potential 28 North Sea Egg Production Irish Sea Hatch SSB 24 Log e Abundance Metamorphosis From : Beverton & Iles (1992) Beverton (1995) Begining Eggs July 20 Start Age 1 Larvae Settlement period Nursery ground 16 Over-wintering PELAGIC DEMERSAL 12 28-Jan 25-Feb 25-Mar 22-Apr 20-May 17-Jun 15-Jul 12-Aug 9-Sep 7-Oct 4-Nov 2-Dec 30-Dec Day of year Nash & Geffen (20??)
Dynamics of a nursery ground Supply of larvae/juveniles Increasing size of individuals Decrease in abundance due to emigration Decrease in abundance due to mortality Time Phases (idealised):
Expected trajectory of log mean weight and log population density Emigration Food limitation? Log mean weight Self-thinning Growth Settlement Log density Inter-annual variations on nursery grounds Differences between nursery grounds High production year High carrying capacity Log Log mean mean weight weight Low production year Low carrying capacity Log density Log density
Plaice nursery grounds with time series of abundance and mean weights
Juvenile plaice: Density versus mean weight Port Erin Bay Po Tr Tralee Bay Expected slope = -1.33 (S (Scotland) Slope = -1.31 Slope = -1.26 Tralee (open) PE Bay (closed) 1986 1987 1000 1000 1988 1000 1989 1995 1996 1997 PE 1995 100 100 100 PE 1996 PE 1997 PE 1998 PE 1999 Mean weight (g) Mean weight (g) Mean weight (g) 10 10 10 1 1 1 0.1 0.1 0.1 0.01 0.01 0.01 0.001 0.01 0.1 1 10 100 0.001 0.01 0.1 1 10 100 0.001 0.01 0.1 1 10 100 Density (m-2) Density (m-2) Density (m-2) Nash et al. 2007 MEPS 344
Other plaice nursery grounds 1000 Swedish Bays 1978 1979 Average weight (AFDW) per plaice (g) 1991 100 1992 Self-thinning 1991 Self-thinning 1992 10 Slope = -1.46 1 Slope = -1.09 0.1 0.01 0.001 0.001 0.01 0.1 1 10 100 Density (per sq m) Filey Bay – density varies x30 Firemore Bay – density varies x6
Drop trap Swedish Beaches (1978, 79, 91, 92) 2 1,5 1 Data 0,5 Thinning Constant biomass 0 Linear (Data) -1,5 -1 -0,5 0 0,5 1 1,5 -0,5 -1 y = -0,778x - 0,3228 R² = 0,7611 -1,5
Dutch Wadden Sea (1975-86, 1991, 1993-2002, 2007, 2009, 2014) 100 10 Mean weight (g) 1 0,1 0,01 0,0001 0,0010 0,0100 0,1000 1,0000 10,0000 Density (m-2)
Varying boundary lines? 1990s 1970s 2 2 y = -1,5681x - 1,4881 y = -1,3426x - 1,8036 1,5 1,5 R² = 0,4154 R² = 0,7464 All data 1 1 2 0,5 0,5 y = -1,371x - 1,6416 1,5 R² = 0,5469 0 0 -2 -1,5 -1 -0,5 0 0,5 1 -2 -1,5 -1 -0,5 0 0,5 1 1 -0,5 -0,5 0,5 -1 -1 0 -1,5 -1,5 -2 -1,5 -1 -0,5 0 0,5 1 -0,5 -2 -2 2000s -1 1980s 1,5 y = -1,1195x - 1,565 2 -1,5 R² = 0,6942 y = -1,7151x - 1,9001 1,5 1 R² = 0,4697 -2 0,5 1 At a given density there was a reduction in 0 0,5 mean weight of: -2 -1,5 -1 -0,5 0 0,5 1 1,5 0 -0,5 55% – between 1970s and 1980s -2 -1,5 -1 -0,5 0 0,5 1 68% – between 1980s and 1990s -1 -0,5 But an increase -1 -1,5 26% - between 1990s and 2000s -1,5 -2 Overall a reduction of: -2,5 -2 80% - between 1970s and 2000s
Comparisons between nurseries – a question Maximum densities observed on the of productivity/differing carrying capacities? nursery grounds 4,0 3,0 2,0 Log Mean weight (g) Tralee 1,0 Swedish DT 1970s PEB 0,0 1980s 2000s Max -1,0 1990s Thinning density Location (m -2 ) intercept Tralee 1,02 14,4 Swedish 0,25 10 -2,0 PEBay -0,23 3 Filey Bay 1,2 Firemore Bay 1,2 -3,0 WS2000s -0,15 0,8 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 WS1990s -0,5 0,8 Log Density (m-2) WS 1970s -0,9 1 WS 1980s -1 0,4
Self-thinning (a single species consideration) A cohort over time, ages and the individuals grow. The resource requirements i.e. space and food increase and thus competition will also increase. The risk of dying increases. If individuals die, density decreases, competition decreases which affects growth – this in turn affects competition and thus growth and so the cycle continues. In animal populations – the ‘rules’ seen in plant populations may be more questionable due to: Variable resource supply Possible other factors e.g. territoriality etc rather and only the food supply Begon et al. 2006 However, they provide a framework for modelling density, with a growth (and thus mortality rates) on nursery grounds, through the summer and autumn months.
The dynamics of nursery grounds can be very complex with numerous inter-specific interactions occurring Predatory gadoids Predatory crustaceans Residents Competitors
In summary: 1. Inter-annual variability in supply of juveniles to nurseries – considerable variability in density 2. Dynamic thinning occurring after settlement ceases 3. Variable slopes for dynamic thinning – often 4/3 4. Mechanism uncertain 5. The influence of emigration on the reduction in density uncertain 6. Upper boundary lines vary between nurseries 7. Upper boundary lines vary over time
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