Observations on Atypical Growth
Observations on Atypical Growth: Rare Events or the New Normal?
David B. Rudders, Sally Roman, Roger Mann and Chase Long Virginia Institute of Marine Science College of William and Mary Gloucester Point, VA
Sea Scallop Stock Assessment Working Group Woods Hole, MA February 5-9, 2017
Scallop Growth Scallop growth is an
import component of assessment models. On average, growth is fairly well understood across the range of the resource although it varies in time and space. Scallop management is predicated on assumptions regarding growth.
Growth under extreme conditions Current assessment/projection
models have implicit assumptions about growth.
Spatial management attempts to
identify areas of high juvenile scallop density, close these areas to fishing and take advantage of fast growth.
We assume that these high density
areas perform according to the resource averages.
Currently, two areas exist in the
resource that contain densities rarely seen in the monitoring time series.
These two areas (NL-S and ET Flex)
currently contain ~50% of non-EFH biomass.
Potential for density dependence Can affect one or
more vital rates (i.e. growth, survival, fecundity).
Effect can get stronger
as population grows and individuals compete for resources.
Where is the
threshold? It is likely complicated.
NL-S and ET-Flex
represent natural experiments
NL-S – high density, small size 5 YO scallops average 77
mm. YOY growth of ~15 mm Large variation in size across space. Can adjust growth assumptions in SAMS model. Reality is that there is a huge biomass of high count scallops with a finite scope of growth in marginal habitat.
ET Flex Another area with
extremely high densities but situated in prime scallop habitat. Similar processes as the NL-S? 2013 YC with an expected SH of at least 100 mm. These animals are driving the biomass in the area.
ET Flex vs. ET Open
SHF for ET Flex (red) and ET Open (blue) 2015-17. 2013 YC appears to have differential growth wrt. area.
ET Blob vs ET Flex 2015 0.20
Blob n= 80,810
Flex n= 78,798
0.15
Mean Length = 48.78 mm
Mean Length = 69.73 mm
0.10
Fraction of Total
0.05 0.00
2016 0.20
Blob n= 61,678
Flex n= 11,780
0.15
MeanLength = 74.47 mm
Mean Length = 87.93 mm
0.10 0.05 0.00
2017 0.20
Blob n= 55,260
Flex n= 6,425
0.15
Mean Length = 80.48 mm
Mean Length = 89.38 mm
0.10 0.05 0.00 0
50 Blob Area
Mean Length Blob
100 Length Interval (mm) Flex Area
150 Mean Length Flex
Comparing the high density vs the rest of the ET Flex area, differential
growth of the 2013 YC is again seen. The area of below expectation growth is the majority of estimated biomass.
Variability at high density Fraction of Total
201501187 0.12 0.10 0.08 0.06 0.04 0.02 0.00
201502109 Mean Length 46.08 mm
201603233
0.12 0.10 0.08 0.06 0.04 0.02 0.00
201503100 MeanLength = 41.49 mm
201704191 Mean Length 76.88 mm
Mean Length 48.37 mm
201704193 Mean Length 68.4 mm
Mean Length 79.2 mm
201704194 0.12 0.10 0.08 0.06 0.04 0.02 0.00
Mean Length 88.49 mm
25
50
75
100
125
150 Length Interval (mm)
For tows >10,000 scallops, there is variability in mean size
especially in 2017. In 2017, all tows were below the expected size for 4 YO scallops.
Digging deeper into the NL Variability at high density• Additional shell samples
(~25 stations) were collected during the 2017 NL dredge survey.
• Objective was to characterize growth across the range of abiotic and biological conditions present in the management area and understand what factors are important in explaining the observed variation. • To date, a small subset has been read. Station 42 84 94
𝐾𝐾�
0.46 0.37 0.42
𝐿𝐿� ∞
151.25 101.1 81.37
• Results seem to support the potential influence of density, but other factors (i.e. depth, food availability) are also likely important.
Summary
Scallop growth, while variable in the NL-S and ET-Flex
(Blob) appeared to be below expectation.
The short term concern is that a large portion of the
total resource wide biomass is contained in those areas. A longer term concern relates to the assumptions of growth, as the management approach and the assessment/projection models are predicated on this understanding.
Has this phenomenon always been part of this species
dynamics and are we just better equipped to observe it? Or…is this something new that we need the capacity to characterize empirically and then incorporate into how we model the resource?
David B. Rudders, Sally Roman, Roger Mann and Chase Long Virginia Institute of Marine Science College of William and Mary Gloucester Point, VA
Sea Scallop Stock Assessment Working Group Woods Hole, MA February 5-9, 2017
Scallop Growth Scallop growth is an
import component of assessment models. On average, growth is fairly well understood across the range of the resource although it varies in time and space. Scallop management is predicated on assumptions regarding growth.
Growth under extreme conditions Current assessment/projection
models have implicit assumptions about growth.
Spatial management attempts to
identify areas of high juvenile scallop density, close these areas to fishing and take advantage of fast growth.
We assume that these high density
areas perform according to the resource averages.
Currently, two areas exist in the
resource that contain densities rarely seen in the monitoring time series.
These two areas (NL-S and ET Flex)
currently contain ~50% of non-EFH biomass.
Potential for density dependence Can affect one or
more vital rates (i.e. growth, survival, fecundity).
Effect can get stronger
as population grows and individuals compete for resources.
Where is the
threshold? It is likely complicated.
NL-S and ET-Flex
represent natural experiments
NL-S – high density, small size 5 YO scallops average 77
mm. YOY growth of ~15 mm Large variation in size across space. Can adjust growth assumptions in SAMS model. Reality is that there is a huge biomass of high count scallops with a finite scope of growth in marginal habitat.
ET Flex Another area with
extremely high densities but situated in prime scallop habitat. Similar processes as the NL-S? 2013 YC with an expected SH of at least 100 mm. These animals are driving the biomass in the area.
ET Flex vs. ET Open
SHF for ET Flex (red) and ET Open (blue) 2015-17. 2013 YC appears to have differential growth wrt. area.
ET Blob vs ET Flex 2015 0.20
Blob n= 80,810
Flex n= 78,798
0.15
Mean Length = 48.78 mm
Mean Length = 69.73 mm
0.10
Fraction of Total
0.05 0.00
2016 0.20
Blob n= 61,678
Flex n= 11,780
0.15
MeanLength = 74.47 mm
Mean Length = 87.93 mm
0.10 0.05 0.00
2017 0.20
Blob n= 55,260
Flex n= 6,425
0.15
Mean Length = 80.48 mm
Mean Length = 89.38 mm
0.10 0.05 0.00 0
50 Blob Area
Mean Length Blob
100 Length Interval (mm) Flex Area
150 Mean Length Flex
Comparing the high density vs the rest of the ET Flex area, differential
growth of the 2013 YC is again seen. The area of below expectation growth is the majority of estimated biomass.
Variability at high density Fraction of Total
201501187 0.12 0.10 0.08 0.06 0.04 0.02 0.00
201502109 Mean Length 46.08 mm
201603233
0.12 0.10 0.08 0.06 0.04 0.02 0.00
201503100 MeanLength = 41.49 mm
201704191 Mean Length 76.88 mm
Mean Length 48.37 mm
201704193 Mean Length 68.4 mm
Mean Length 79.2 mm
201704194 0.12 0.10 0.08 0.06 0.04 0.02 0.00
Mean Length 88.49 mm
25
50
75
100
125
150 Length Interval (mm)
For tows >10,000 scallops, there is variability in mean size
especially in 2017. In 2017, all tows were below the expected size for 4 YO scallops.
Digging deeper into the NL Variability at high density• Additional shell samples
(~25 stations) were collected during the 2017 NL dredge survey.
• Objective was to characterize growth across the range of abiotic and biological conditions present in the management area and understand what factors are important in explaining the observed variation. • To date, a small subset has been read. Station 42 84 94
𝐾𝐾�
0.46 0.37 0.42
𝐿𝐿� ∞
151.25 101.1 81.37
• Results seem to support the potential influence of density, but other factors (i.e. depth, food availability) are also likely important.
Summary
Scallop growth, while variable in the NL-S and ET-Flex
(Blob) appeared to be below expectation.
The short term concern is that a large portion of the
total resource wide biomass is contained in those areas. A longer term concern relates to the assumptions of growth, as the management approach and the assessment/projection models are predicated on this understanding.
Has this phenomenon always been part of this species
dynamics and are we just better equipped to observe it? Or…is this something new that we need the capacity to characterize empirically and then incorporate into how we model the resource?
