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Shellfish Catch Trends in Massachusetts and Buzzards Bay: Trend Analysis using an Imperfect Data Set

Sub-pages:   Shellfish Trends Buzzards Bay vs MA  

by Dr. Joe Costa

Page last updated: September 30, 2016. The spreadsheet on this page (posted in 2012, with data through 2011) is being updated with commercial and recreational catch data through 2015 for Buzzards Bay municipalities (where available). We are also correcting some past data entry errors, and improving the spreadsheet functionality. The "read me" tab in the spreadsheet includes important information about limitations and omissions of the data.

Background: The original data set of municipal shellfish catch and permit data used on this page was prepared by the Massachusetts Division of Marine Fisheries (DMF) about 2006, with some additional updated information collected by the BBNEP, and included in the 2012 original posting. The data was provided to the Buzzards Bay NEP by Neil Churchill, who maintained the original data set. Information about the data set is available at this DMF website. In 2015 and 2016, the Buzzards Bay NEP received additional data from DMF that was based on their new online reporting system for commercial fisherman. The updated dataset will include data from this new dataset. Any use of this data set should credit the Massachusetts Division of Marine Fisheries (DMF). DMF can also provide additional information and guidance on the use of this data sets. This data set may contain errors, and individual municipal catch and permit data can also be independently corroborated through review of municipal annual reports, available at municipal libraries, although local and state data may contain inconsistent data reporting standards, such as summaries by calendar or fiscal year. Use of the Shellfish Landings Relative Trends Analysis Tool Spreadsheet at the bottom of this page should acknowledge this webpage.

As noted on the DMF website,"[DMF] is charged with collecting, analyzing and maintaining an historical database of commercial and recreational shellfish landings. This information is initially collected by each of the 65 coastal cities and towns of the Commonwealth and submitted annually to us on "Town Landings Forms". Data collected reflects the number and types of permits issued, the pounds of each species landed and by what shellfishing methods. Along with data, the municipalities submit updates of their local shellfishing regulations. This data has been maintained since 1955 in both hard copy and electronic format and is used for fisheries management on the local, state and federal levels."

Commercial Shellfishermen report their catch with this commercial shellfishing reporting form. Most coastal municipalities in Massachusetts annually submit Town Landings Forms, which includes estimates of recreational permit catch. There is variability in how towns estimate recreational catch, with some towns estimating catch by daily counts of recreational shellfishermen, other towns basing it on survey sheets, or some other methodology. An example of how some towns collect recreational catch data is illustrated by this Chatham 2009 shellfish permit form. However, there is variability in how the data was recorded, and certain permit data may not have been consistently recorded. For example, some towns issue scallop-only permits, and these permit counts may not be included in reporting sheets for each year.

When evaluating historical trends with this data, methods for estimating recreational shellfish catch may have changed over time, or when there were changes in municipal personnel. Some of the limitations of this data set are contained in this MA Ocean Plan chapter on living marine resources. Moreover, not every municipality reports this data each year, or may omit estimates for certain species. The most objectively accurate information contained in this data set, when reported, is the actual number of commercial and recreational permits issued by a town.

Data for 48 towns are listed in the data set. The greatest utility of this data set are the town specific summaries of catch and permits over time, and for making relative comparisons of catch volume across regions, or comparing catch statistics between municipalities for a given period.

The data becomes problematic when all municipal data is aggregated, because relatively few towns are represented in the data for the 1950s and 1960s, and many municipalities are not reporting their catch data to the state, especially in recent years. The purpose of this webpage is to address this single confounding element of this data set: missing data. We undertook this effort because we wanted to better characterize shellfish catch trends in Buzzards Bay as compared to elsewhere in Massachusetts.

The DMF data set is especially useful in tracking trends, like permit data in specific municipalities as in the case of this Town of Bourne data.

Town data and relative catch volume

Because of data omissions, it is best to evaluate trends by reviewing single municipal trends, or creating a set of indicator towns where data is consistently available during the period of interest. However, this approach discards some valuable relative trends for municipalities where the data is less complete. When data is aggregated across all towns, most species will display a graph like the one below left: Low totals during the 1950s and 1960s, a somewhat noisy trend in subsequent years, then dramatic declines in the past decade. This trend, however, is primarily reflecting the fact that few values exist in this data set for the 1950s and 60s, or after 2006, as illustrated by the graph, below right.

The apparent trends shown by total municipal softshell clam catch in the graph at the left are largely due to the fact that few town reports are included in the data set for the 1950s, 1960s, and few towns reported data to the state after 2006.

In an effort to standardize or normalize this data set, the BBNEP added a new field (column) to calculate the relative ranking of each town's species catch for the entire 1955-2008 period to create a quantity-independent index of trends among all municipalities where data is available. For every year, the shellfish catch for that species was normalized by the average catch of that species in that town. For example, if the long term average catch for that species in a town were 1,000 bushels, and in a specific year, the town catch was 2,000 bushels, then the index value for that year would be 2.0. By taking the average of all these relative ranks for each town and year (excluding missing data), statewide trends over the 53-year period become more apparent as illustrated by the graph below. [Note: an alternative ranking could also be achieved by dividing each year's catch by the best year catch, but there is little additional benefit offered by this approach.]

The advantage of this approach is that towns with few data points can be included in the analysis in a meaningful way to better characterize long-term statewide trends. The chief disadvantage of this approach is that towns that issue few permits or have modest shellfish catch are weighted in the analysis equally with towns that have a large shellfish catch. However, the strong correlation in the graph below, and others, suggests that there is considerable commonality of trends among the municipalities in the Commonwealth, at least for some species. In these relative trend graphs, for any particular year, values above 1 means the totals or statewide average was above the long term relative averages. Values below 1 means the year averages or totals were less than the long term average.

In this graph, each town's soft shell clam catch was indexed against that town's average catch for all years where data was available (town year catch divided by town average catch for the period). Then, for all years and all towns where a catch was reported, these relative index scores were averaged to create a statewide relative index. From this approach, a clear trend emerges showing that in most towns, some of the best catch years were in the 1950s, 1960s, and 1970s, with catch often dramatically declining since that time. For this analysis, data for at least three towns had to be available for a point to be plotted. In the figures, values above 1 means average catch was above the statewide long term relative average. Values below 1 are less than the long term average.

Click to see more trends.

Massachusetts Shellfish Landings Relative Trends Analysis Tool Spreadsheet

This processing of the DMF data set on this page was undertaken with Excel lookup tables, array formulas, index functions, and dynamic graphs to minimize time-consuming brute force manipulations of the data set, and to quickly generate the data presented here. The key formulas are explained in the ReadMe tab of the spreadsheet.

Please note that the calculation methodology utilizes array formulas. These are identifiable by the {curly brackets} surrounding the cell formula. To preserve the array formula function (and not break the calculations), when editing these cells, make sure you press CTL-SHIFT-ENTER after making any changes in the cell function. There is also some use of the Relative Function, which means that the spreadsheet could also be broken by inserting rows or columns in certain key areas of the spreadsheet.

Shellfish Landings Relative Trends Analysis Tool xlsx Office 2010 Spreadsheet last update: October xx, 2016. [The spreadhseet is being updated, it will be re-posted sometime in October 2016]. Right click to download, and then open. If your browser renamed the file ".zip", rename the file to the ".xlsx" extension. If you only have Office 2003, download MS Office 2007 compatibility pack to open this spreadsheet.

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