Environmental monitors on lobster traps (eMOLT): long-term observations of New England's...

Environmental monitors onlobster traps (eMOLT):

long-term observations ofNew England’s bottom-water

temperatures

J Manning, Northeast Fisheries Science Center, National Oceanic and Atmospheric

Administration, Woods Hole, Massachusetts, USA

E Pelletier, Gulf of Maine Lobster Foundation, Kennebunk, Maine, USA

Nearly one hundred New England lobstermen have installed temperature sensors on

their traps to record hourly values at fixed locations since 2001. These moorings are

distributed primarily along the shelf edge in the northern Mid-Atlantic Bight and along

the entire western edge of the Gulf of Maine in a range of water depths (1–300m).

Variability associated with tidal, wind, seasonal, and inter-annual processes can be

depicted at nearly all sites. Tidal variation, for example, at certain times of the year in

many locations can be significant (.108C). Wind forcing is shown to significantly modify

the seasonal cycle at many locations such as in Massachusetts Bay where a dramatic

turnover occurs in the Fall. Inter-annual anomalies are derived by removing seasonal

cycles. Comparisons between sites and between years are made. The years 2002 and

2006, for example, are documented as warmer in general than other years at nearly all

sites. While a direct correlation between temperature variability and lobster catch is

difficult to quantify in the data collected thus far, preliminary investigations document the

relationships on a seasonal time scale.

The possibility of incorporating this network of moored sensors into a regional ocean

observing system is addressed and the limitations are discussed. Given the minimum cost

required to deploy the instrumentation and the sustained interest of the fishermen, the

initiative provides a means to collect data continuously and a strategy for monitoring

environmental change on climatic time scales.

AUTHORS’ BIOGRAPHIES

James P Manning has been employed as an oceanographer

for the NOAA’s Northeast Fisheries Science Center for

more than 20 years. After receiving his masters degree in

oceanography from the University of Rhode Island in 1987,

he has been investigating physical processes on New Eng-

land’s shelf and their relationship to fisheries. In addition to

working with local fishermen, he is particularly interested in

providing data to local coastal ocean circulation modellers

to help initialise, assimilate, and validate numerical simula-

tions.

Environmental monitors on lobster traps (eMOLT)

Volume 2 No. 1 2009 Journal of Operational Oceanography 25

Erin Pelletier has worked on eMOLT and a variety of other

collaborative research projects for the last decade. As ex-

ecutive director of the Gulf of Maine Lobster Foundation,

she coordinates, for example, extensive ‘ventless trap’ and

‘rope exchange’ programs with fishermen throughout New

England. Trained in environmental education, she specialises

in outreach to fishermen and helps them sustain their

resource.

INTRODUCTION

Much of the historical record of bottom tempera-

ture in the New England region comes from

NOAA’s Northeast Fisheries Science Center1

and Canada’s Department of Fisheries and

Oceans.2 These observations extend back nearly a hundred

years so that decadal scale variability can be investigated.

Since most of these observations are shipboard casts made

a few times per year, the higher frequency event-scale

variability in temperature is largely missed. Beginning in

2001, the Gulf of Maine Ocean Observing System (Go-

MOOS) deployed moorings at several locations that mea-

sure temperature throughout the water column. These

observations provide greater insight into higher-frequency

temperature variability but only at a limited number of

locations – limited by the cost of mooring operations.

There are several million lobster traps deployed around

the Gulf of Maine. The fishermen who maintain these

moorings could make a significant contribution to the ocean

observing systems of New England. Not only are they on

the water on a near-daily basis for much of the year but

they have a sincere interest in their resource and the envir-

onmental changes that may affect that resource. The Envir-

onmental Monitors on Lobster Traps (eMOLT) project has

collaborated with lobsterman and deployed internally-

recording temperature sensors on lobster traps on a routine

basis since 2001. Each fisherman typically has one or two

probes deployed at a fixed location (fixed depth) that are

recovered on an annual basis to download the hourly time

series. The database of bottom temperature from over 100

locations has now grown to over three million records,

which allows investigation of temperature variability from

tidal to inter-annual time scales. Since shipboard protocol

often calls for randomly-selected station positions, bottom

temperature from fixed locations and depths is rare in the

historical record.

The primary objective of the eMOLT system, like many

ocean observing systems, is to provide numerical circulation

modellers the data they need to properly initialise, assimi-

late, and validate their simulations. Given the multiple-

scales of variability in both temperature and salinity, it is

increasingly clear that model runs need data to provide

realistic fields.3 While tremendous advancements have been

made in recent decades in Gulf of Maine circulation model

accuracy4,5,6,7 success is limited by the difficulty in properly

resolving the fine-scale baroclinicity of the density fields.

After a brief description of the eMOLT data and the

methods used to process the data, the patterns in the data

are described according to time scale. An analysis of the

inter-annual variability is followed by the tidal variability,

and finally the event-scale process of ‘turnovers’ is pre-

sented. In some cases, the GoMOOS data is used and com-

pared with the eMOLT data. Since much of the results are

presented in the way of examples and since it is often

necessary to focus an analysis in specific regions much

smaller than the entire Gulf of Maine, the Massachusetts

Bay area is chosen as a sub-region, although the same type

of analysis can be conducted throughout the study area.

While not the focus of the report, the biological impli-

cations of bottom temperature change are presented in the

discussion. The eMOLT project has also attempted to docu-

ment the relationship of lobster catch to both the absolute

temperatures and the variation of temperature over time. A

small portion of the eMOLT participants contributed to this

aspect of the study by providing catch data along with their

temperature records. While this aspect is not the focus here,

examples of the analysis are presented and discussed in

relation to other studies of a similar nature.8,9,10

DATA/METHODSWhile the protocol has evolved over the years since the

project inception, the general procedure is simple and now

involves the steps outlined in Table 1.

The primary sensors used in the study thus far are the

ONSET Tidbit and the VEMCO Minilog which both have

resolution and accuracy better than 0.28C. Depending on

the range of temperatures expected in some regions, some

probes are specifically engineered for smaller ranges and

can therefore resolve temperatures near 0.18C. In some

cases, the LOTEK LTD-1100 and SEABIRD Microcats

were used.

A few years into the eMOLT project, lobster catch data

was recorded by several of the participants on the Thistle

electronic log books but, after a year or two of successful

operation, the company that sold the unit dissolved. While

• The fishermen attach the sensor to the lobster trap for the entire duration of the fishing season

• The science party mails a self-addressed-stamped envelope to the fishermen near the end of the year asking for:

a) probes to be returned

b) documentation on any changes in mooring location/depth

c) catch for traps with probes attached (optional)

• The data is downloaded, cleaned, filtered (for occasional spikes associated with hauling), plotted, archived, and posted on the web

• Probes are immersed in a controlled ice-bath to check on biases and calibrations

• The science party reinitialises the temperature sensor at hourly sample rate and mails it back to participants along with hard copy plots of

the data.

Table 1: eMOLT Procedures

26 Journal of Operational Oceanography Volume 2 No. 1 2009


there has been some effort by the scientific community to

continue the use of these loggers, most participants reverted

to hand-written logs. Since the data collection was con-

ducted on a voluntary basis, only a few dozen participants

provided catch records. Some documented in detail the

numbers of legals, shorts, and eggers, while others recorded

legals only. More than a thousand hauls have been recorded

to date and, while this phase of the eMOLT project is no

longer funded, several participants continue to submit their

catch records.

Each temperature time series is processed by truncating

the records for ‘in-water’ periods and subjecting the rest to

a range and delta check. The range and delta check criteria

varies depending on the region and a visual inspection of

the raw data. Beginning with a conservative test of all

temperatures between �6.6–26.78C and any changes of

0.88C/h, the tests are iteratively performed with a graphical

inspection at each stage until only those spikes likely asso-

ciated with hauling the instrument are removed. Plots of

raw, filtered, and daily averages are plotted for each record

and posted on the web. Processed data is served with Open-

Source Network Data Access Protocol Standards at http://

emolt.org.

The most important protocol that has been stressed

throughout the programme is maintaining fixed sites. Since

it is often impossible for lobstermen to land the traps on

exactly the same bottom with each haul, we expect ‘fixed’

sites are those which vary by ‘no more than one mile and,

most importantly, no more than 5% of the water column

depth’. Given the vertical stratification in many locations, it

is important to maintain this protocol. In some locations

such as the edge of the continental shelf, this is particularly

important. In most of these shelf edge locations, fishermen

are supplied with a pressure sensor along with the tempera-

ture sensor (VEMCO Minilog-TDR) so that a rough esti-

mate of water depth becomes part of the data stream at

these ‘fixed’ locations. These pressure sensors are accurate

enough to register changes greater than 5% of the water

column. Errors associated with trap movement to multiple

depths are addressed further in the discussion.

While each fisherman typically installs temperature

probes only on the trap at the bottom, several have also

attached a sensor 1m below the surface buoy as well. In

these cases therefore, a continuous record of thermal strati-

fication is available.

The organisation and outreach for the project is

provided by the various lobstermen associations in New

England. The four largest groups (Atlantic Offshore, Massa-

chusetts, Maine, and Downeast) have all had at least several

participants involved from the start of the project. They

provide updates and reminders to the participants via near-

monthly meetings, newsletters, and annual forums. An up-

date on the project is presented at all the annual forums. An

eMOLT newsletter is mailed to all participants a few times

per year with plots of the most interesting findings.

Temperature probes, administered by the eMOLT pro-

gramme, are now being deployed on a set of projects

around the Gulf of Maine including multiple ‘ventless trap’

programmes and a ‘lobster settlement’ programme. In both

cases, biological data is kept in a systematic way so that

correlations with temperature may be possible in the near

future. These parallel studies are conducted by both federal

and state agencies in the US and by the Fishermen and

Scientist Research Society (FSRS) in Canada. Beginning in

2006, several eMOLT probes have been deployed by Nova

Scotian fishermen. Temperature records from various pro-

jects are stored in the same database and served at http://

emolt.org.

RESULTSThe geographic distribution of eMOLT sites, presented in

Fig 1, covers the majority of the Northeast US Continental

Shelf but is limited to those areas where lobstermen fish.

Fig 2 depicts the distribution of eMOLT records as a func-

tion of water depth where approximately a third of the sites

are in depths of 90 fathoms (165m) or more with a large

portion near the 45 fathom (82m) depth. The distribution of

eMOLT temperature records over time is presented in Fig 3.

While the majority of sites have short records, over 100

sites span multiple years.

Interannual variability

Given several years of data from various depth zones off

the coast, it is now possible to quantify seasonal cycles and

calculate typical deviations. The range of temperatures ob-

served in 2006, for example, versus those observed through-

out the eMOLT record (2000-present) is presented in Fig 4.

The time series of the anomaly (seasonal cycle removed) is

depicted in Fig 5 for four distinct sites. For the first time,

we now have an objective measure of continuous bottom

temperature relative to other years in various depth zones.

A traditional shipboard observation in October of 2006

would have registered 2006 as being a normal year while

one in late November would register it as a very warm year.

We know from the continuous record, the anomaly changed

sign within a few weeks time. Extracting the monthly

Fig 1: Multi-year eMOLT bottom temperature mooring

locations



means, it is possible to look at a simple time series of the

interannual progression (Fig 5). It is still too early to claim

specific long term trends in the eMOLT records but we

have noted that 2002 and 2006 as being, in general, the

warm years.

Tidal variability

Hourly observations adequately resolve a semi-diurnal tide

in nearly all the eMOLT records. In a few cases, the ampli-

tude of the tidal signal is equal or greater than the seasonal

or wind-driven signals. In some locations, the sensors are

evidently placed in the vicinity of the seasonal thermocline.

Fig 6, for example, comes from Alex Brown who fishes the

inside of Cape Cod. Here the tidal oscillations can exceed

108C during the passage of a front and quickly drop to near

zero variance within the time scale of an upwelling/down-

welling cycle. The vertical stratification as observed by

Billy Anderson off Lubec Maine can vary as a function of

the lunar cycle by as much as a few degrees (Fig 7). Two

days subsequent to the timing of each full moon during the

summer of 2007, the increased tidal velocities were appar-

ently sufficient to mix the entire water column in that area.

Massachusetts Bay turnovers

Sensors at the bottom of Mass Bay for the last several years

have documented a dramatic turnover each fall. The records

from 2005 (Fig 8), for example, show the degree to which

the water column can mix a very stratified water column

the matter of a few days. A wind event that occurs in early

October 2005 was sustained from the north for multiple

days and was able to mix the entire bay to within a few

degrees C. Sensors in shallow water and near the surface

recorded cooling temperatures while the deeper sensors

recorded a warming event. In the case of fishermen in deep

waters, Dave Kandrick (DK) and Rob Martin (RM) for

example, the bottom temperature increased by nearly 58C

during the event. Those fishing in relatively shallower

waters closer to shore like Bill Doherty (BD), saw tempera-

Fig 2: Distribution of eMOLT sites relative to water depth

Fig 3: Distribution of eMOLT records over time

Fig 4: Example comparing 2006 (solid line) to long-term

mean (dashed) and min/max (gray) for the case of the 26

eMOLT sites off the coast of Central Maine in the depth

zone (0–20m)



ture rise several degrees in the matter of one tidal period.

This difference indicates a downwelling scenario rather than

a direct wind mixing event. It is interesting to note that

these turnover events due to particular storms register at

many sites all along the coast. The effect of the storms is

often depicted at nearly all regions as shown, for example,

in the Fall of 2006 at multiple sites (Fig 9).

DISCUSSIONThere is a variety of limitations associated with collabora-

tive research with fishermen. As noted in the methods

section above, there is uncertainty in the mooring location

and depth. Since the trap movement often happens without

clear documentation, the temperature time series is often

affected and the scientific investigator is required to take

this uncertainty into account. With enough moorings in the

region of interest however these errors can be detected and

filtered from the dataset. In other words, since temperature

time series at nearby locations are often coherent, one can

determine artificial ‘events’ associated with trap movement

from real events associated with wind, tide, and river run-

off.

Another difficulty in working with New England fisher-

men is the fact that they typically work non-scientific units.

Their positions are most often reported in ‘Loran’, depth in

Fig 5: Four examples of

anomalous temperature

time series after having

removed the seasonal

cycle



‘fathoms’, time in ‘local’, and temperature in ‘Fahrenheit’.

The investigator needs to take care in properly converting

to more scientific standards but, at the same time, be

prepared to deliver the results back to the fishermen in their

terms. The majority of eMOLT graphical results therefore

are presented in the units requested by fishermen, the pri-

mary stakeholders.

While there is plenty of anecdotal information about the

affect of temperature on lobster catch, very little has been

quantified. Given multiple years of data, we can now begin

to investigate the relationship. In Fig 10, for example, it

appears that the seasonal catch curve is similar although

slightly lagging that of temperature. However, the tempera-

ture vs catch relationship evidently depends on the time

scale, the season, and the location. Where we find catch

still increasing with decreasing temperatures in the fall,

Drinkwater et al10 found the opposite in the spring. Their

results pertained to shallower waters far to the north of our

study area. When Worden et al8 used eMOLT data to

investigate the changes in haul-to-haul catch, no significant

relationship with temperature was found in this less-than-

seasonal time scale. The ventless-trap studies now have a

growing database of catch from several years. Given their

more-controlled data collection system, it is hoped that

these catch vs temperature relationships can be refined in

the near future. In the preliminary correlations of eMOLT

catch data with temperature, the relationship for legal-sized

adults vs that for ‘shorts’ and ‘eggers’ may be different.

The investigator needs to distinguish between the various

life stages.

While we have focused here on the first phase of

eMOLT project (temperature and, to a lesser degree, lobster

catch), it should be noted that the eMOLT project has

evolved to also include observations of salinity and cur-

rents. Several lobstermen have deployed Seabird Micro-

cats11 (Fig 11) and satellite-tracked drifters.12 In eMOLT

Phase VI, some are deploying a recently-developed bottom

current meter. As described at http://www.emolt.org, the

Fig 6: Example of tidal

and wind-driven

variability in Cape Cod

Bay over two-week

period



project continues to evolve as new instrumentation is devel-

oped that will allow telemetered real-time observations by

satellite, radio, and cell phone. It is hoped that the eMOLT

concept can be incorporated into the regional coastal ocean

observing system both in the Northeast US (ie, NERA-

COOS) as well as other regions of the world. The primary

step in ‘integrating’ the eMOLT system with others in the

region, providing data via a distributed server, has already

been accomplished. As suggested by labs participating in

the Gulf of Maine Ocean Data Partnership, the eMOLT data

is served.

SUMMARYResults to-date demonstrate a variety of processes that were

previously difficult to document given the limited number

of moored and shipboard observations. Tidal variability, for

example, has been shown to be significant in many loca-

tions where the amplitude of the signal can be greater than

several degrees Celsius. The dramatic effect of winds and

the ‘turnover’ event that occurs each Fall in places like

Massachusetts Bay is better defined. Lobstermen can, for

the first time, say for certain just how warm or cold it is

relative to the previous years of observations. The years

2002 and 2006, for example, were documented as 2–38C

warmer than other years.

While a definite relationship between temperature and

lobster catch is still difficult to parameterise, records show

an obvious coherence in the seasonal cycle of both variables

with catch lagging the temperature in many cases. The

highest catches apparently occur days-to-weeks after the

water begins to cool in the Fall.

Dozens of New England lobstermen have been monitor-

ing bottom temperature on their lobster traps since 2001.

This dataset provides time series for investigating the ef-

fects of tides, wind, seasonal heating, and offshore influx in

a variety of locations ranging from the Hudson Shelf Valley

to the Nova Scotia. In the future, as the programme con-

Fig 7: Example of variability related to the lunar cycle

Fig 8: Example of wind-

induced ‘turnover’ in

Massachusetts Bay in

2005



tinues, it will provide a long-term record for documenting

decadal scale climate variability. Given the low-cost instru-

mentation and the interest and involvement of the fisher-

men, it is hoped that the programme will be sustained and

be merged into whatever observing system develops in the

Northeast US.

ACKNOWLEDGEMENTSThe authors thank the Northeast Consortium for several

years of collaborative research funding. They also appreci-

ate the help of Bonnie Spinazzola from the Atlantic Off-

shore Lobstermen Association, Jeremy Cates and Clare

Grindal from the Downeast Lobstermen Association, Patrice

McCarron from the Maine Lobstermen Association, and

Dave Casoni from the Massachusetts Lobstermen Associa-

tion for making eMOLT work, and, of course, the dozens of

New England lobstermen for providing their time, insight,

and effort.

REFERENCES1. Mountain DG and Jessen PF. 1987. Bottom waters of

the Gulf of Maine, 1978–1983, Jour. of Mar. Res., 45:319–

345.

2. Petrie B, Drinkwater K, Gregory D, Pettipas R and

Sandstrom A. 1996. Temperature and salinity atlas for the

Scotian shelf and the Gulf of Maine. Canadian Tech Rep. of

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3. Pringle J. 2006. Sources of variability in Gulf of

Maine circulation and the observations needed to model it.

Deep Sea Res. II. 53(23–24) p.2457.

Fig 9: Example of coastwide effects of moving storm affecting

Massachusetts Bay to Eastern Nova Scotia. The Nova Scotian

sites are not shown in Fig 1 since they are not part of the

multi-year eMOLT set

Fig 10: Example time series of temperature and catch

Fig 11: Seabird Microcat mounted inside a lobster trap.

[Photo by Norbert Lemeiux, lobsterman from Cutler Maine]



4. Lynch DR, Ip JTC, Naimie CE and Werner FE. 1996.

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the Gulf of Maine, Cont. Shlf. Res., 16, 875–906, 1996.

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